An HD 2600xt 512mb can run Crysis on medium at 1280x800 with 37fps and costs the same or less than an 8600gt. While the HD2600xt performs better than a 8600gt in Crysis it does not perform better in many other games. You should really try to get an HD 2900 Pro. It is more expensive than a 8600gt or HD2600xt but it will perform a lot better.
Because i don't want to - and my friend has loads of the 8800's - Ultra's and all. Just I am not going to be able to have 1. Of the two choices I am going with the 8600GT.
Where are you buying from that only gives you the choice of those two cards? Because there are much better UK shops than wherever you must be buying from.