Depends on whether you want a DirectX 10 card or not.
PCA recommends this one click here just a touch over £250 but if you shop about you might get it cheaper. And this one was number 5 click here but only supports DirectX 9.0 unlike the BFG card.
Depending on what you want it for you might not have to spend all that much. The ATI 1950 Pro Ultimate for example is only £136 click here but again DirectX 9.0 only.
Remember to check you power supply is up to it.
Have a look at this site which lets you compare various cards click here