i think you should go for the pre-built system that marine suggested because you seem unsure of components and probably how to put together nicely. better to get it professional built and for that system i would for for the gtx285, windows 7 Pro x64 and then with the budget you stated of £2000 you got over £500 for a…
keep in mind that i7 = triple channel ram, so those who suggest 8/16 gigs with an i7... Not doable. You can do 3, 6, 12, or 24 DDR3 with an i7 CPU. I think 12 is a good number, but 6 would be very usable as well(i have a quad core with 8 gigs, very nice system). A budget of £2000 seems quite high, for that price you should…