Here is another approach. Do a bare bones upgrade. I deal with a local computer reseller that is fairly big for being in rural Illinois. I tell the owner what I'm going to do with the computer and what processor I want to use (Athlon in my case) and he sells me what works for him with the hunderds of computers he assembles each month. In other words what's stable.
Just recently he spec'd out a athlon xp1600, a Epox motherboard, 256 DDR ram and a standard midtower case (Enlight) with 300watt power supply. It cost me $460.00 for those components. He attached the processor and heat sink/fan to the motherboard and "burned it in" (ran it for like 6 hours). I put the motherboard/cpu into the case plugged in the ram dragged over my other current components (hard drives, cd burners, video card, sound cards etc) and booted up. No problem at all. Now I have to switch out the noisey components (hard drive, power supply, cpu fan/heat sink, case fan) but I can do that over time. You can also rebuild your old computer with extra or used parts to make a nice home network email station, storage center, or internet gateway.
The benefit in going this way is that you are using your computer reseller's hands on experience while you do the assembly. And since you bought the components from him (or her) you can go to them for some tech support if there is a glitch.
good luck
jack