There are two choices...either an NVidia 6600GT (around £100) or a 7600GT (around £140).Look on the web though because you should be able to find either cheaper than those rough guideline prices.
Performance wise the 6600GT is slightly better than a 9800xt card so around 50% faster than your 9700pro.
The 7600GT is much faster still and will double the performance of your 9700pro at least......when used at high resolutions that is.
the importeant thing is to bear in mind the 7600GT is easily the best mainstream choice for playing any forthcoming games.
Incidentally what motherboard did you get?....I need to update my graphics card but like you have a socket 478 cpu......it might prove cheaper to change my own motherboard and use a pci express graphics card than upgrade to one of the silly priced AGP cards (EG Gainward 7800GS goes like hell editions).