Just had a look on toms hardware and the 8800GT performs alot better with higher resolutions.............
Not sure on prices though i havent checked.
You cant go wrong with a Q6600 GO stepping, its just picking the right graphics card.

EDIT just had a look on Ebuyer, your looking at 138 quid for a 3870 512mb, and about 150 quid for an 8800GT 512MB.