Wait for HD5850 If you're going to game at that resolution for another games like Crysis, Stalker or Far Cry.
You have like 1000USD.. Um..
How does the prices differ to you than in the USA as I can't reach eBuyer for some reason.
£1200 in USD is £2000...as far as pricing is concerned if a Intel Core i7 860 2.8GHz Socket LGA 1156 8MB L3 Cache Retail Boxed is £219 here it's $365 in US. (as in £219 = $365 not i7s in US cost that...I just converted it)
I used the Over Clockers site for the fact it's a lot easier to manuever. You can go to ebuyers and match up the products on this list...this is just the lay out of the build. Anyways this is the build for what you wanted/stated. This board comes with o/c utilities and I included the o/s in here.
http://www.ebuyer.com/product/161702 £154.98 inc vat Free Delivery
ASUS GTX275 896MB DDR3 Dual DVI HDTV Out PhysX and Cuda ready PCI-E Graphics Card Includes Batman Arkham Asylum download & 3D Discover Glasses