GPU: Sapphire 4870 @ £180, I don't see the point of Asus.
However, 4850/ 4870 is a bit hot. You may want to change the heatsink later.
CPU: C2D is a better option for gaming...
RAM: DDR2 is OK. Also, by the time DDR3 RAM drop price and DDR2 become expensive, you'd probably build another PC already.
HDD: I don't see the point of spending £200 for RAID0 with WD VelociRaptor, I personally will go for one...
PSU: The one you choosen seems expensive. There should be other
good quality PSU with a lower price.
780W is more than you require at the moment, but when you consider crossfire later, 700-800W should be the right choice.