I think only the AGP card that can fold on the current client is the ATI 3850 (or any of the 3xxx series and above, but i dont think there are many agp cards), and to be honest, its output is pretty dismal.  At a guess it may do a few thousand ppd if you are lucky, but others may know more.  I dont think it really justifies the cost unless you can find one cheap.  I dont think nvidia do an AGP card that will fold, but it needs to be an 8 series or higher card to run CUDA.  Sounds like a good excuse for an upgrade to me!
What sort of CPU have you got...?  Folding on a quad/dual is pretty good for points and probably a better use of leccy than an AGP card.