Let’s hear the details: MB, CPU, hard drives, video cards, water cooling components… Everything.
I’d buy the HP Blackbird 2 which last I checked cost “only” about 6K, but had pretty much top of the line components.
I’d buy parts one year behind the curve for a total of $500 for the system and then pocket the $9500.
Oh, but you can’t, though. 
Because you just gotta have the best 'cause you’re a crazy-ass gamer, man!
Well, it’s not only about the box, is it?
I have my homebuilt computer hooked up to a 40" Samsung LCD and I don’t think I can ever go back to a normal sized monitor. Then there’s sound. What kind of speaker system should I run? I currently have Logitech Z-5500 but it’s nowhere good enough. If I had $4k to blow on speakers, I’d get the DALI ikon6.
Apart from that, water cooling is overrated. I have a case from Antec, with two 120mm fans for the case and a CPU fan from Noctua. The HD is louder than the cooling, so water cooling is not gonna help, but it’ll be messier to build.
Zalman used to have a case w/o fans or water for about $1k, but their website is wonky right now, a friend of mine who bought it complained about the loud noise from the HD.
As for CPU, MB, videocard, soundcard, DDR2 or DDR3, well…
It’s easy to pick the most expensive product available. But how do you notice the difference between Intel Core 2 Quad Q6600 quad-core processor, 2.40GHz, 8M L2 Cache, LGA 775 ($300) and Intel Core 2 Extreme X6800 Conroe 2.93GHz 4M shared L2 Cache LGA 775($1k)? Well tom’s Hardware shows a roughly 25% increase in performance, depending on application. But is it worth 3x the price?
That’s up to the individual user.
For myself, I’d rather spend the $10k on paraphernalia (sp?) than internal hardware and eye candy (LEDs, water, tribal paterns on the fans), but then again, I’m a 46 y.o. potbellied geezer
What Just Some Guy said, though maybe I’d go balls-to-the-wall and blow a couple grand instead of just $500. The problem is that the state of the art in PC gaming hardware advances so fast that almost no matter how much money you spend today, your system is going to be pretty much middle-of-the-range in a few years, and it goes downhill from there. PCs may be some of the only products that depreciate faster than cars.
In a lot of ways - and this is where people are going to start throwing things at me - game consoles are a better gaming investment than PCs. Since the hardware is “fixed”, and new consoles don’t come out that often, developers tend to spend a good number of years making games for that console
I’m with Charlie Tan on this. Ars Technica’s system guide for the God Box looks like more power than I will ever use. I’d be more tempted to go with the Hot Rod hooked up to a 36" display and spend $1000 on an Aeron chair (another Aeron, I guess, since I alread have one) than putting it into the system itself. Computer hardware just ages too fast.
God Box link: Ho-Ho-Holiday System Guide: DIY for the holidays | Ars Technica
I’ll be your token Mac dweeb for this thread (cuz it’d r0xx0rz when it isn’t playing games):
Subtotal $9,479.00
Specifications
Two 3.0GHz Quad-Core Intel Xeon
8GB (4 x 2GB)
750GB 7200-rpm Serial ATA 3Gb/s
750GB 7200-rpm Serial ATA 3Gb/s
750GB 7200-rpm Serial ATA 3Gb/s
2 x NVIDIA GeForce 7300 GT 256MB
Apple Cinema HD Display (30" flat panel)
One 16x SuperDrive
Both Bluetooth 2.0+EDR and AirPort Extreme
Apple Wireless Keyboard and Apple wireless Mighty Mouse - U.S. English
Mac OS X - U.S. English
I too am a Mac Dweeb. 
Specifications
Two 3.0GHz Quad-Core Intel Xeon
8GB (4 x 2GB)
Mac Pro RAID Card
750GB 7200-rpm Serial ATA 3Gb/s
750GB 7200-rpm Serial ATA 3Gb/s
750GB 7200-rpm Serial ATA 3Gb/s
1 x ATI Radeon X1900 XT 512MB
Apple Cinema HD Display (30" flat panel)
Two 16x SuperDrives
Both Bluetooth 2.0+EDR and AirPort Extreme
Apple Wireless Keyboard and Apple wireless Mighty Mouse - U.S. English
Mac OS X - U.S. English
Total (with Educational Discount): $9,638.00
You’ll note I added the hardware RAID card to increase performance, two superdrives (for twice the (legal) Music/DVD ripping goodness), and went with the single X1900XT graphics card over the dual 7300GT’s.
Really, I’d like to go with a card other than the X1900XT but until apple offers more choices, it’s the best card to be stuck with for a gamer while still maintaining the compatibility needed to run OS X.
I have the Dali Ikon6’s, and i love them and all, but holy crap, that’s insanely overkill for just a gaming rig.
Scratch that. Instead of getting the memory from Apple, I’d go with the stock memory (and not use it), add a second 30" monitor instead, cut the Superdrives back down to one, go wired keyboard/mouse, then buy 16GB of RAM from Newegg for a total of 756.91 (with shipping). Total cost now?
$9962.91
Geez, all that money and in a few weeks, it won’t be the “new hotness” anymore. Intel is coming out with they Penryn processors which Tom’s Hardware overclocked to 4.00Ghz with air cooling and AMD’s quad cores are coming out soon (though I think TH also reviewed those and they aren’t that fast).
Besides, do that good games even run on a mac?
No one for twin 30" displays?
I debated buying the ram from Apple, and not keeping up on the graphics cards, just took a stab in the dark. I don’t think apple does the ‘SLI GPU’ overloading thingy, do they?
I still find it funny the best Vista laptop’s a Mac.
With that kind of money to spend, I’d almost certainly opt for a server board mainboard and I’d make sure it had 4 CPU slots (all quad-core) and about 16Gig capacity RAM (It’s possible that I’d have to go with XEON technology to pull this off, though I hear SuperMicro has a board that fits the bill here). It would also need to support 2 16x PCI-Express (running in 2x16 mode) with 8800 GTX Ultras and an aftermarket cooling system. The rest would go toward the best monitor(s) I could get.
I know that all that CPU power is wasted with today’s games that are only coded to take advantage of dual core CPUs, but I’m also a 3D artist and those applications would take FULL advantage of that setup.
An Imac and a used car, or an Imac, booze and hookers. In fact, forget the Imac.
Well, if the gaming rig is part of your home entertainment system, connecting the dvr, dvd player, gaming console…
They do from bootcamp. Then it’s just a PC doing PC things in a pretty case.
Ars Technica regularly updates the specifications on their God Box. The late 2007 version will cost you $14,000 and comes with eight CPU cores and 16GB of RAM. You can probably shave it down to $10k if you ditch the 4TB of spare hard disk space, the second graphics card, and the second 30" monitor, but where’s the fun in that?
$500 will buy you a functional computer, but in terms of cutting-edge gaming systems that would be more like 4 or 5 years behind the curve. Hell, a GeForce 8800 Ultra video card alone costs more than that.