Depending on how much other hardware you are going to run, I would go with 700w for starters. If you run more hardware, then you could go up but for the most part I think that you should go with 700 watt power supply.
I'd think 650 or 700 watts would be plenty. I run 2 4870s, 4 HDDs, a Q9650, and 5 fans with 700 watts. Corsair, Mushkin, OCZ, Seasonic, Thermaltake, XIGMATEK, and Silverstone are all good brands. Here's a calculator that may help you decide. It says I need 900 watts
You shouldn't need more than 750W from a quality PSU unless you plan on going SLI. It's better to look at the amperage on the +12V rail than the combined max wattage.