Assuming your PSU is a decent branded one and at least 450w, then the best graphics card at the moment on AGP is a GeForce 7800 GS OC 256MB see click here for an idea of price.
If you fancy ATI, then there are still some ATI Radeon X850 XT's around. See click here for examples of price etc.
ATI on the way out (as I expect you realise) so unlikely to be that many new ones coming out.
Of the two mentioned, the Nvidia is the more powerful.