whats the difference between PCIe 2.0 and PCIe 1.0 and can i run a 2.0 card in a 1.0 slot and what would be the best graphics card for my system
AMD Athlon 64 x2 dualcore processor 4200+ 2.2GHZ
NVIDIA 8400 GS is the current card
Other than bandwidth, there no difference between pci-e 1.0 and 2.0.
As for can you run a 2.0 card on a 1.0 slot? Yes. PCI-e slots & cards of both versions can run in any of the configurations. http://en.wikipedia.org/wiki/PCI_Express
As for what gpu to get. Kind hard to say. You are going to lose a little bit of performance by not running a pci-e 2.0 card on a 2.0 slot, but the cpu is going to be more of the bottle neck.
Also what games are you playing?