I'd recommend going with a single rail PSU with that setup given any potential load balancing issues - unless you can work out exactly how to balance dual rails (1 rail to motherboard powering CPU, RAM, and 75W x 2 for the PCI-E slots for the graphics card and any other PCI peripherals ...ok that's alot of demand on a single rail)
Seasonic and Tagan both do excellent single-rail PSUs. Tagan is the one I can vouch for - the 530W unit has 32A on the +12V (and 48A on +5V)
Out of interest why aren't you going for a 7800GTX given its faster than SLId 6800s, is less expensive and draws less power? If I had a 6800GT I'd sell it and buy a 7800GTX if I really anted that sort of graphical power.