Mate, if your motherboard has AGP slots then thats all it will take, end of. Unless you get a whole new motherboard with pci express slots.

As for the card id go for the nvidia, i used to have a 256mb ati radoen x1300pro and i swapped it for an nvidia geforce 7600GT and its miles better even though its still 256mb

Heres some in-game screen shot from my pc