You have a few choices for a budget of £100.
If you game at high resolutions (1900*1080) & above try go for the >512Mb versions of these cards. Otherwise 512Mb will be sufficient. The 1st card ideally needs 2x dedicated PCI-E connectors, while the others require one.
In order of preference:
1. HD 4870 (~£90 e.g. click here)
2. GTS 250 / HD 4850 / HD 4770 (~£75-85 e.g. click here, click here & click here)
3. HD 4830 (~£65 e.g. click here)
Best 4850 for the money (£83) is this Gainward 'Golden Sample' click here, & one with arguably the best cooler, this Sapphire Vapour-X click here. But a HD 4870 will be the best if you have a sufficient PSU for it. G