Hi Cadelin
There are rumours of a 9800 GT coming out in the summer, which will replace the 8800 GT. However, don't expect them to be at or below £100 when they launch.
Given that your computer is two years old, either an 8800 GT or 9600 GT should offer a nice performance boost (the latter is not quite as powerful as an 8800 GT, but it has better HD video decoding, better DirectX 10 performance, and a fairly small price tag). Always check out some reviews on the internet before you buy
Kind regards