AGP or PCI-E? <- very important compatibility detail!
Hopefully you'll be PCI Express, in which case I'd highly recommend the 512mb x1950 pro at ebuyer.com: http://www.ebuyer.com/UK/product/119303
I know it's £5 over, but it's massively better than anything below £100, and it's only about 50p more expensive than their cheapest 256mb version, so it's bound to be on offer at the moment. Also delivery is free, which you won't get on most sites.
Make sure your motherboard has a PCI Express x16 slot first tho! If not then you'll need an AGP card.