current advice for gamers on a budget and with an AGP board seems to be an Nvidia 6600gt, just over £100 at the moment. You could even find a 6800 vanilla for not much more if you shop around.
If you prefer ATI, their X800GTO is quite a bargain since it's now superseded by the x1800 (it doesn't have shader model 3 like the 6600 / 6800, but I doubt that makes any difference in today's games)
There's an Ice-Q version with an excellent cooler.
Whatever you go for, just make sure your power supply is up to the job; spare molex connector for the card, 350-400W and a decent manufacturer.