Best gaming rig for £500 would look like this
imo
Intel E5200 (no need for quad for gaming) ~£55
4GB PC6400 (OCZ is fine) ~£50
DFI Lan Party P35 ~£75
640GB Western Digital AAKS ~£50
Antec 300 ~£35
Corsair 450W VX ~£45
ATi HD4850 ~£120 (sapphire dual slot) or to use the whole budget the Nvidia GTX260 (might want a different PSU for this).. that is what i would buy but it depends what resolution you wish to game at
Samsung DVD-RW ~£15
Total is about £500
Don't skimp on PSU's or the Case, or the motherboard which isn't very good for stability (the 650i that is)
I'd also use
www.scan.co.uk. Better prices for the most part and great service.