Well if expense is not a problem Mar 22nd sees the release of Nvidia's GTX 590 which at a guess will be the fastest single card (dual GPU) on the planet
As to your PSU I'd be looking at bigger than 750 watt...PC's are becoming more power hungry as time goes by, graphix in particular. I would be considering at a minimum, a 1000 watt psu thats capable of supporting Multi GPU set ups. Or bigger even...PSU's tend to be swapped out of and into new systems so future proof yourself
And be sure to get a motherboard thats supports SLI/Xfire dependent on what card set up you go for.