600 GBP? That's pushing it in terms of crazy features, but lets see.
For gaming, there is no substitute for the video card. Focus on the video card, a reasonable CPU and everything else is secondary.
For that budget, the Radeon X800xl might be the best fit after you get the rest of the parts together. Maybe an X800XT or a Geforce 6800 Ultra if the budget allows. With a bit of overclocking, my x800xl will handle Far Cry at maximum settings, zero AA and 1x AF and a resolution of 1280x1024 with a good 50ish fps.
For the CPU, I'll reccomend an Athlon64 3700+ (the model with 1mb cache)
Mobo, DFI Lanparty UT nF4 Ultra-D
Ram, anyting PC3200 or faster. Realistically speaking, you can go with run of the mill PC3200 ram and get great performance while saving a great deal of cash. Just don't expect to see any good overclock results on your FSB though.
Everything else - just go with what your budget allows. Just don't skimp on the power supply. Get a branded one like an Antec or OCZ Modstream.
