£600 to £800 gaming pc

PSU: 650W Antec Signature Modular 80plus £82 - Very good PSU which should give you plenty of headroom for overclocking. And crossfiring, which the motherboard supports if you ever developed a desire to do this. I personally don't see a need to downgrade either of these components to remove the possibility of crossfiring, as both these components are quite good and worth the money either way,
HDD: Samsung Spinpoint F3 1TB £70 - One of the best harddrives around.
RAM: 4GB (2x2GB) Corsair XMS3 Dominator DDR3 PC3-12800 (1600) CAS 8 £117 - Overpriced RAM, but fast and low-ish latency. If you want RAM that is not overpriced you would have to look at a site other than scan.
MOBO: MSI 790GX-G65 £93

I make your total with my suggested parts to be £722

I'm not too sure on what would be the best CPU cooler to get, but it's not absolutely essential. The suggestion that I would make is for a heatsink that I haven't found on scan.

A review about the Antec PSU: http://www.jonnyguru.com/modules.php?name=NDReviews&op=...
Some reading for you about the motherboard:
http://www.legitreviews.com/article/953/1/
http://www.overclockersclub.com/reviews/790gxg65/21.htm

thanks very much for your help