For around £130 you can get a Geforce 6800, or a 6600GT. I'd go for the 6600GT - performance is roughly the same, the 6600GT has DDR3 and the 6800 has fewer pipelines compared to the 6800GT versions. This one looks reasonable, and silent too
If the heatsink on that one looks too cumbersome, this one
does the job with a small fan. If you're willing to spend about £50 more you could get a 6800GS, which is as good as a 6800GT but quite a bit cheaper. £185 on that site but might find it lower elsewhere.