Im big on multitasking,(downloading something,converting audio and video,surfing the web,messenger,watching tons of movies(big movie fan),gaming and well the computer I have right now is nowhere near what I need and I mean not even close.
I also have a multicard reader, plan on buying another hard drive (another 500gb maybe more), planning on getting an HD moniter (I watch a lot of movies and series) and I'm going to get a Bluray burner.
The general rule of thumb is a high quality 500 to 550 watt power supply can easily handle a computer system with a single video. Corsair, PC Power & Cooling, and Seasonic all have reputations for high quality psu's that are stable and reliable.
Antec Earthwatts 500W or Truepower New 550W for a little more headroom. If you're sure about wanting SLI later, get an Earthwatts 650 or Truepower New 650 (or 750 of either) which will be able to run two cards later.