I have a P4/1.8 ghz PC. The highest I can upgrade the processor is 2.8ghz , for about $200 or so.
The only problem is that the mobo only supports up to a 400mhz FSB. I know that P4’s have mostly been 533mhz FSB’s for a year or two, and now all the new ones are 800mhz.
What “real world” affect does this have on your PC’s performance? I do a lot of audio recording and editing, if that matters.
Lots.
Although some of it is misleading and I am by no means an expert.
FSB is how fast everything else talks to the CPU. It matters a lot how fast things get a chance to access the CPU. What’s misleading is the MHz rating. RAMBUS 800 MHz FSB for instance isn’t that fantastic when you consider “slower” DDR 400 MHz ram transfers 8 bits of data across the FSB instead of 2 bits. This is why DDR400 has a higher data transfer rate than RAMBUS (Intel is really bent about MHz ratings even though in this case slower is faster).
RAMBUS RAM makes the baby Jesus cry. 
Quite important (check the wikipedia article on front side bus), so a motherboard change is required for the higher FSB. However, you can do the upgrade to a 2.8GHz P4 with your current MB – check out www.powerleap.com for options. That’s what I did some time back.
Amen to that.
FSB is very important. Many (if not most) consumers groove to the Mhz (or Ghz these days) rating of their CPU thinking that a blazing fast CPU=blazing fast computer (it does not although it is a part). I am the computer guy in my family and get family and friends coming up to me all the time saying they saw a 2.8 Ghz computer for sale for $800! What a dope I am to have paid more for the PC I built. One even made me benchmark my PC versus his “faster” CPU and got quiet real fast when mine proved to be nearly twice as fast despite his “faster” clock speed.
The CPU is certainly the most important component in a computer but it is reliant on many other things to work up to its potential. When the CPU does not have the instructions it needs it checks its cache (on-chip memory these days) for the instructions. If it cannot find what it needs there it checks main memory. While this happens your CPU will idle till it gets what it needs. A good FSB is like putting an Indy race car on a racetrack. A bad FSB is like having an Indy race car in rush hour traffic in Chicago. Big deal if your car is capable of 200+ mph speed if it sits still more than it moves.
Computers use predictive algorithms to try and “guess” what they will need before they need it and make sure it is available so the CPU avoids idling and surprisingly these algorthms do a fair job of it but they still get it wrong plenty of times.
All of that said bottom line is what you plan to use the PC for. If it is just for e-mail and word processing and playing MP3s the $500 PC will do just fine. Those programs are easy enough on today’s processors that it would be hard to notice the difference between a bleeding edge fast PC and a mediocre one. If you get into video editing, advanced graphics programs, games, etc. then the faster computer starts making a lot more sense.
Its not all there is to it.
I play some 3D games that are pretty taxing. My old PC was an Athlon XP 1900 (~1600 mhz I think). Battlefield Vietnam FPS was dropping below 10 in some maps if I was in a tank, so I upgraded video card first to a Radeon 9800 PRO. Helped some, but not a huge difference between the 8500, and this didn’t surprise me. I was bound in CPU. Upgraded CPU to P4 3ghz and the performance increase was phenomenal. Usually see FPS in the 80-100 range, never seen it drop below 70.
But dig this, I still have one stick of PC2100 ram. So my FSB is choked way back and the performance is still fine. I’ll probably upgrade it to 3200, but right now I really don’t need to.