Something is not right there. $1500 should get you a machine that can do more than a little gaming. $500 should get you a machine that can “do some gaming”.
1500 better have 8 gigs of ram, dual vidcards and a fast quad processor, not to mention a solid state drive at least for the OS and a raid array of hard drives.
The main point here is that with the modern systems, there’s not a lot of parts needed. As mentioned above, the video, network, drive controllers, ports etc. are all pretty much included on the motherboard. So basically - motherboard, processor, power supply, DVD writer, disk and RAM.
If you want high power (fastest processor, or it will be a server doing virtualization and you need many cores in processor); or you want a really fancy video card or something - it’s far more efficient to customize only the parts you need. The el cheapos often come with DVD reader of writer for example, and they charge you well above the price differential at some retailers - or you have to customize after you buy and throw the old part away. Or you have only X slots for memory, and they are full of small RAM modules.
While OEM legally -according to MS - limits you to the original system that Windows came with, I have never heard that principle proven in court. Curious - has anyone heard of it?
Anandtech gave them a pretty solid review at that price point:
*In this case, pricing the components at Newegg actually came in around the sticker price, so these services essentially come for “free”…
This corroborates what we stated in our Holiday Guide - around the $1500 mark, the bang-for-your-buck here is very high. Unless you simply enjoy the process of putting together your own PC, even enthusiasts would be happy with what CyberPower offers.*
From here: CyberPower Core i7-920
They offer a fair amount of customization at a pretty decent price and since I get no joy from building my own systems, I went with them for my own “mid-range” system 
. I do agree that after pricing out heavier duty systems I think you can save rather more on the truly high-end by building it yourself. But low to mid-range ( i.e. sub-$2000 ), some of the boutique companies like cyberpower seem awfully compeititive.
That IS a good deal, but it’s also one hell of a system. It’s not a “I can do some gaming system” It’s a "I can rock Crysis at super high resolution while it gives me sexual pleasure system 
this.
If you want a gaming computer with 4 hard drives in a mirrored/raid configuration then you build it. It’s not to save money but to get what you want.
It includes Windows 7, i5-750 processor, monitor, Office, etc. The only thing in my current machine that could be reused is the case.
I also threw in a few extras.
This is not the final configuration, I was comparing DIY and being built by the seller so just came up with a system to compare. I do need to research cost/benefit ratios on everything.
I was the system admin for a hotel and we had hundreds of computers and as each one failed, I would grab parts that still work and make my own computers. It was cheap and fun.
Could you do that now? Sure but as others have stated, the mass production means it most likely wouldn’t be cheaper.
[Slight hijack]
I have an old computer (6+ yrs old) that worked fine up until the power supply ZAP!ped out. The workings were fine, except the hard drive was a bit small by today’s standards. How easy/practical would it be if I could just slap in a new HDD and power supply and fire it up? I imagine I’d have to load Windows in there, but would it be advisable?
Tripler
. . . or, should I just stick with what I have and add components like a second HDD?
You should be able to plug in a replacement PSU and be good to go, especially if it’s a standard ATX one. That said, a failed PSU can have all sorts of nasty knock-on effects.
How do you mean “knock-on” effects? I imagine a weird power spike, or a short, could scramble up the 0s and 1s on my HDD. . .
Izzat what you mean? 'Cause I live in Utah now. An old desktop makes a great rifle/pistol target. 
Tripler
Just curious. . .
An option no one has mentioned yet is a local mom and pop computer shop. They tend to be price competitive (or they won’t survive), can get you anything you want within a couple of days, and charge on the order of $20 to $30 to assemble the system.
Let’s not forget the one thing you can’t do yourself-the warranty.
You can also do what I’ve generally done, and simply upgrade piecemeal, eventually replacing everything but the case, several times. Particularly if you started out getting a good full tower case which will take the extended ATX form factor, and cards of any height. My Addtronics case originally came with a system from one of those custom integrators. I’ll agree that that is a good way to go if you’re buying a whole system - you just tell them what you want, and they don’t charge that much for assembly. As well as saving you some work, should something prove defective, they just go get another one out of the back room, while you have to hassle with returning it to get a replacement. At any rate, I believe that case has contained 3 or 4 motherboards and 2 power supplies since. If I were going to replace the current motherboard, I would retain the HD and video card which are both newer and perfectly fine for a newer motherboard. The power supply is a 470W Enermax which should be suitable for anything I could currently envision wanting to put in there.
I had the same situation you do recently, and found out after I bought and installed a new PSU that the motherboard got fried when the old PSU died. I THINK and hope that the HDD is still intact but that’s uncertain since I don’t have an enclosure to check it out and haven’t gotten hold of my friend who does.
Most of the parts you purchase come with a warranty. The Intel processor I purchased for my build came with a 3 year warranty from Intel. The video card a two year warranty. The ram came with lifetime warranty. The hard drive a three year warranty. Etc. Building it your self seems to come with a better warranty than buying from Dell. I only got a one year warranty for that.
Pretty much everything I bought for my computer had a warranty. Plus you’re not getting the worst possible power supply ever when you build it yourself. A good quality high end supply makes things so much nicer.
Not only that, but there are hundreds of horror stories on the web about people who send their computer in for warranty service, only to have it take weeks or months to come back. In a few cases, it disappears altogether.
If you build your own, and can just send back the faulty part, you don’t risk losing the whole computer, with all your information.
Related and rather heated thread in The Game Room. Despite the heated arguments, I found it quite informative.
Another advantage to building it yourself is that, as yabob mentions, you can do it piece by piece instead of all at once. Some things have to be replaced together, like the motherboard, processor(s), memory, and with some companies, the case, but things such as drives, monitors, and input devices are mostly universal and can be transferred over until you get to them. This computer has cost me around $3000, but the most I spent at once was around $800.
Yet another advantage is that it’s easier to upgrade it later on. Like I said, I’ve spent around $3000 on this machine but I started building it about 4 years ago. These dual Opteron 244 processors aren’t very fast by today’s standards but other parts are still very desirable. This motherboard will support the newer dual-core Opterons (Italy architecture) though so I could replace them, add more memory, and get a pretty good boost. You’re less likely to be able to do that with a prebuilt system.
And finally, it can be useful to know exactly what’s in your system and that’s much easier to do if you built it. I can tell you pretty much exactly what’s in this system, but not my laptop.