I'm making the assumption here that your friend already has software otherwise he is going to have to compromise. This ought to be suitable though, you have a fast CPU which is very overclockable, a decent graphics card and solid sundry components.
MO-016-OK OcUK Gamer Extreme 178GP 17" LCD Monitor - Black (MO-016-OK)
£91.99 £91.99
CA-052-AN Antec NSK4400 Mini Tower Case - 380W SmartPower PSU (CA-052-AN)
£43.99 £43.99
MB-064-GI Gigabyte GA_945PL_S3 (Socket 775) PCI-Express DDR2 Motherboard (MB-064-GI)
£57.99 £57.99
MY-059-CS Corsair 1GB DDR2 XMS2-5400C4 TwinX (2x512MB) (MY-059-CS)
£69.99 £69.99
CP-135-IN Intel Core 2 DUO E6300 "LGA775 Allendale" 1.86GHz (1066FSB) - OEM (CP-135-IN)
£107.99 £107.99
HS-017-AR Arctic Cooling Freezer 7 Pro CPU Cooler (Socket 775) (HS-017-AR)
£17.99 £17.99
GX-025-BG BFG 3DFuzion GeForce 7600 GT 256MB GDDR3 TV-Out/DVI (PCI-Express) - Retail (GX-025-BG)
£74.99 £74.99
HD-023-SA Samsung SpinPoint P HD080HJ 80GB SATA-II 8MB Cache - OEM (HD-023-SA)
£25.95 £25.95
CD-020-SO Sony DDU-1615 DVD-ROM (Black) - OEM (CD-020-SO)
£10.50 £10.50
Subtotal £501.38
Shipping (City Link Parcel Next Day (Delivered Mon-Fri)) £12.95
VAT £90.01
Total £604.34