If you have the moenym get an AGP version Nvidia 6800 - they are great cards, you can usually pick them up for around 150quid
However if you can't afford that much, I would advise against getting a low to middle range card, and instead get a top of the range but older card second hand, for example a radeon 9700pro can be won on ebay for around 30 quid, and will far out perform a brand new 6200 or 7200 and still be half the price...
Just my 2cents though I kow people get very territorial when graphics cards are discussed around here
My advice, do your research before you buy a model that "looks" like it'll do the job