For your money i'd suggest looking at a 1GB 4670 which should hit your pocket for between £60-£70. A 512mb version would be fine, but depending on the resolutions you intend to run it at, more memory is better. It will also give you a year or two before requiring upgrading again.

Have a look here at scan's website, they do some of the best prices (just order online! Queuing up there takes hours )

www.scan.co.uk

It really depends on what resolution you're running and what settings you want.  A Radeon 4830 will run at around 1680x1050 high settings buttery smooth, and costs around 65 pounds.  The 4670 mentioned above will be a little bit cheaper but much slower, I wouldn't save that money for that much of a performance drop personally.  If your running only 1280x1024 than it might make sense to go that route.. If you can find a Nvidia 8800gt or 9800gt for a similar price, it would be close in performance to the 4830, perhaps slightly better. 

You might be able to get a used Radeon 4850 within that range if you feel you badly need something better.

Oops... I mixed up posts here... Edited to make more sense.