DIY PCs are cheaper in most cases.

opteron 170 939 dual-core all the way, what's quadcore for... it's cheap (older socket), fast, giant cache, and it's AMD -> 110€ comes with excellent cooler
socket 939 mainboard -> 40-100€
2 Gig RAM -> 160€
2 samsung SATA HDD -> 200€
enermax power supply -> 60€
case (if you don't want to use tzhe old one): 30-50€

this is around 550€ + GFX board of your choice, around 200€. so you don't need to exceed 750€ for a really good PC.

use old DVD/CD drives, soundcard, firewire card etc.