Thanks a lot guys, this is really helpful.
As for a few questions raised:
-Will be buying parts separately and assembling myself, not pre-built.
-No I will not OC until the warranty has run out on CPU/GPU.
-Large power consumption isn't too much of a problem. PC isn't always turned on and my current one is 750W.
-I would rather go for Intel + Nvidia as that's what I'm familiar with (not to mention PhysX
)
Undecided whether to recycle parts from my old PC or not. As far as I know the Dell XPS 720 case is good but that'd mean I wouldn't be able to sell it on. I think it's only the HDD failing in my PC as I'm not getting any BSODs etc. just corrupted files, frequent automatic chkdsks and messages such as 'no boot device available - F1 to retry'. If that is the case I can sell it on without the HDD, the parts by themselves aren't worth much.
And yes I think I was very lucky with this PC. Bought it for £200 in Dec 2010. Q6600 and 8800GTX which must be the top of these phases you talk of, because they're still going now a lot better than I thought they would be.
As for CPUs, is there much of a difference between i5-xxxx and i5-xxxxk? And any advice on 3570k vs 4670(k)?