How to build a PC on the cheap
I built my brother this PC last year using
http://www.aria.co.uk (if you know how to build a PC)
1 DVI-D to DVI-D Cable 2 Meters £ 2.89
1 LITEON DH-20A4P 20x DVDRW - Black £ 12.65
1 Intel Pentium Dual Core E2180 2.0GHz (Retail 775) £ 43.15
1 Antec NSK4000 Blk/SilverTower £ 24.75
1 Xilence Red Wing Case Fan 8cm £ 1.95
1 XFX GeForce 9600GT 512MB PCI-E £ 98.35
1 250GB Seagate "Super Quiet" DB35.3 IDE 8MB £ 33.95
1 MSI PT890 Neo-V Socket 775 MOTHERBOARD £ 22.95
1 Kingston 2GB PC2-4200 ValueRAM (2x1GB) £ 23.25
UK delivery was £20 for this lot plus VAT was around £284
It plays crysis on high setting and any steam game you can throw at it but you might want to get a better CPU fan than the stock one, I suggest arctic cooling freezer 7 pro which is also on the site for £11. The case has a side panel vent with a CPU funnel but I guess the case is really up to you. You might also want to fork out for a bigger HDD but I was going for the cheap option for my bro (I'm not that rich)
I didn't include keyboard and monitor, you can get a switch box if you wanted and have both mac and PC hooked up to the same peripherals. My sister uses this for her set up: Belkin KVM / Omniview 2-Port switch but it only does VGA cables.
Prices will have changed since then so you could probably build a better PC than this for the same price.