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

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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

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And be sure to get a motherboard thats supports SLI/Xfire dependent on what card set up you go for.