As long as you don't need the OS then it can be done (if you do need an OS learn Linux

). This should give you a very capable gaming machine with some potential to upgrade but really you would need to upgrade the motherboard before considering much overclocking etc as it is fairly basic.
NEC AD5170 18x18 DVD±RW Dual Layer ReWriter (Silver) - OEM £16.99
(£19.96) £16.99
(£19.96)
Asrock Conroe945G-DVI (Socket 775) PCI-Express DDR2 Motherboard £43.99
(£51.69) £43.99
(£51.69)
Corsair 1GB DDR2 Value Select PC5300 Dual Channel Kit (2x512MB) (VS1GBKIT667D2) ( £71.99
(£84.59) £71.99
(£84.59)
Western Digital Caviar Special Edition 80GB 800JD SATA-II 8MB Cache - OEM £26.99
(£31.71) £26.99
(£31.71)
OcUK Value Hanns-G HC174D 17" LCD Monitor - Silver/Black £99.99
(£117.49) £99.99
(£117.49)
Sapphire ATI Radeon X1950 Pro 256MB GDDR3 HDTV/Dual DVI (PCI-Express) - Retail £107.99
(£126.89) £107.99
(£126.89)
Antec NSK4400 Mini Tower Case - 380W SmartPower PSU £42.99
(£50.51) £42.99
(£50.51)
Logitech Cordless Desktop EX-110 - Retail £17.99
(£21.14) £17.99
(£21.14)
Intel Core 2 DUO E6300 "LGA775 Allendale" 1.86GHz (1066FSB) - Retail £105.99
(£124.54) £105.99
(£124.54)
Sub Total : £534.91
Shipping cost assumes delivery to UK Mainland with:
City Link Parcel Next Day (Delivered Mon-Fri)
(This can be changed during checkout) Shipping : £12.95
Vat : £95.88
Total : £643.74