pci-e SLI's not an option for the future with +1x16 and +1x8 and +1x4; look for a board with 2 full x16 slots, aka x32 "pci-e SLI ready" For future proofing towards an sli upgrade...

You may want to reconsider the motherboard, and videocard; Find a board that will support ati-crossfire and look at the new ati cards.

As for the Power Supply, high end computers need a specific "amperage" rating for specific voltage's

The PSU needs to be able to handle certain number of amps/watts to power the video cards, ontop of the rest of the system..

For a decent CPU/Videocard, hard drive/optical drive, you want something like a solid 520-800 watts, for sli you're looking at 800+. Voltage stability is key to overclocking> if you are overclocking you want the overhead of a 1000 watt PSU so that the voltages do not drop too much while the videocards sucking power... The whole system may not actually draw 1000+ watts, but it will be able to handle peak loads without voltage drop's, which might cause system instability etc...