minge, as you have a motherboard with an Nvidia chipset, my advice would be to go with an Nvidia graphics card.
From what I've read on the Nvidia forums( click here ), some people have problems getting ATI cards to perform to their best when installed in a system with an Nforce chipset. Also you should find an Nvidia graphics card will perform(slightly)better when used with an Nvidia motherboard.
If you go for an Nvidia card, for the amount you're willing to spend, a 6800GT would be your best bet and can be had for around the 200 quid mark, depending on which brand you go for.
Here's a couple click here and click here . The XFX card's a really good price when you factor in you get a copy of Doom 3 with it.