Cost-wise the 7900 GT is amazingly affordable for the second-fastest solution from Nvidia, as you can pick up the NX7900GT-T2D256E for about £237 online, although stock seems to be short at the time of writing. There will be tough competition from ATI, and you could get an X1800 XT for similar money, but this is a much noiser card.
MSI might not be the first company you'll think of if you were to get a new graphics card, but it has been in the graphics card business for quite some time now and has a decent track record. And with top-of-the-range Nvidia cards there's generally little that differs from one to the next, unless you're going for an overclocked card.
The MSI NX7900GT-T2D256E is the first 7900 GT to make in to the Reg Hardware lab and it's an affordable high-end graphics card which won't disappoint. You can get much faster cards, both from Nvidia and ATI, but then you have to add another £100 or so to the price tag.