You want good performance in games and the best value?
CPU: Phenom II X4 940
Any 780G motherboard, doesn't matter.
4GB DDR2-800
AMD Radeon HD4870 1GB
Performance: Highest settings on resolutions up to 1920x1200.
Yeah, PCs are cheap these days. You can quadruple the cost of that PC by getting Core i7 CPUs, dual graphics cards, DDR3 RAM and an X58 mobo per above, but that won't get you better performance except on insane resolutions like 2560x1600.
EDIT: Realised you're British so removed US prices.







