AGP isn't going to last very long and you really should get a new PCI-E motherboard and the new 6800 GS. But if you don't want to wait till you can get PCI-E just get a 6800.
What is the socket that your CPU uses? If you have a CPU that uses the PCI-E motherboards you should just wait till you can get a new mobo and get the 6800GS which is almost as good as the GT and is only 209 bucks but if you don't have one that uses new mobos then you should just get a 6800 or get a new proccessor and mobo then get the GS.