Ok, so heres some ideas i've been putting together, but i'm by no means a 'Gaming' expert...theres a few bits missing and it's currently a bit over budget but...
4GB Corsair (2x2GB) TwinX XMS2, DDR2 PC2-6400 (800), 240 Pins, Non-ECC Unbuffered, CAS 5-5-5-18
Intel Core 2 Quad Q8200, Yorkfield Core, S775, 2.33GHz, 1333MHz, 4MB L2 Cache, 7x, 95W, Retail
Asus P5KPL SE, Intel G31, Sok 775, PCI-E (x16), DDR2 1066(OC)/667/800, SATA 3Gb/s, ATX
1TB Hitachi Deskstar 7K1000.B, 3.5", SATA 3Gb/s, 7200 rpm, 16MB Cache, HDD
750W Powercool 80+ Dual 12v Rail V2.2 High Efficiency Black Psu
896MB XFX GTX260 XT, 2000MHz GDDR3, GPU 621MHz, 216 Cores, 2xDL DVI-I/HDTV + FarCry 2
That little lot comes to about £500...no screen is required for now, just the case and inners...so who knows what I should be buying and swapping from the list above? Has to be £500 or lower for a fully functioning gaming PC