Agree with saidur_ali on the most part.
Best CPU right now for the money is the i5 2500k.
It must be said though for a budget rig an athlon ii 640, phenom ii 955 or even a decent dual core can run most of the games out there as they're mostly GPU dependant and a CPU costing £70-100 has no probs. Still these are ageing CPUs and AMD should've replaced these by now. There's only a handful of CPU demanding games that could trouble these.
A GTX 570 running Metro 2033 on Very High will only get to around 40FPS whether you use an Athlon or a 990X overclocked to 5.0GHz. You'll need two graphics cards, not a powerful CPU.
Running at low res to lesson the GPU load so very high frame rate is achieved or low demanding games like Resident Evil 5 with a decent GPU can mean 120Hz monitor can benefit as the frame rate will 100-200FPS. An Athlon may bottleneck the GPU to 80FPS. Using an i5/i7 will let the GPU do its thing.
Say your budget is £600 and you want to play crysis etc. Spending that extra £100 on a i5 2500k instead the of the GPU and getting a cheaper GPU like a GTS 450/ATI 5770 instead of a GTX 560 and a cheaper CPU/mobo for example would be very bad. You should get a Phenom and the best GPU you can.
The 6990 GPU is out soon. A Phenom 955 with a 6990 will wipe the floor with a i5 2500k and a GTX 580 in Crysis/Metro 2033, yet both will cost a grand. The i5 2500k will beat the 955 in video editing and many other tasks etc, playing Starcraft 2, getting 300-400FPS in a console port or a synthetic benchmark for e-peen.