Seeing how you have an AM3 motherboard, an AMD CPU will probably be close to bottlenecking two 6950s even a fast CPU.
But that motherboard has the space to install those.
Any good power supply in the 700+ watt range can power the cards, you said CPU but sounds like you meant GPU. This is a list of the AMD certified power supplies, http://support.amd.com/us/certified/power-supplies/Pages/listing.aspx
You need a PSU with 4 75 watt video card connections, or an adapater.