Question Best pcie 2 graphics card

[Bladestorm1412]

I have a 7 year old PC where the graphics is the bottleneck. I know the best advise is to start from scratch and build a whole new pc but I simply don't have the money. I'm looking for the best pcie 2 graphics card and the specs of my ancient PC is:

AMD phenom 2 quad core over clocked to 4.7ghz
Asus crosshair 4 formula
8gb Kingston memory

Is it viable me buying a pcie 2 graphics card?

 

[Bladestorm1412]

Furthermore I just read that pcie 3 cards does work in pcie 2 motherboards so is it better buying a pcie 3 graphics card?