There is no "requirement" for PCIE video cards. They are all compatible from 1.x to 3.x. Secondly, take a look at the Radeon 7750. It uses no external power, draws all the power it needs from the PCIE slot and can be found for under $100 (after rebates). No need to upgrade your PSU.
Such as the Sapphire Radeon 7750 1gb
$95 shipped $85 after MIR