Build a need computer Watts?

First off the main thing you will need to understand, don't look as much at Watts. Look more at amps. In other words, you see a power supply, it will have numbers listed for it's specifications on the sticker on the side. It should have a reading for the 12v+ rail. There can be more than one of these. But then below that it will give a number. For example the number could be 18, 20, 24, etc. This is the number of amps for that rail. The more amps, the better the power supply probably is. One thing you will find if you look at cheap power supplies, a lot of them say they have a lot of watts, but then when you look at the amps they supply, it's a very low number in many cases.

Good explanation Ohiou_grad-06 You'll also need to be aware of the PCI-e power connectors for you video card. In your case, you'll need 2 x 6 pin PCI-e power connectors to hook up your card correctly, so your PSU will need at least 2 x 6 pin PCI-e power connectors. Here's an example picutre of what it looks like on the video card.
I'd go with at least the Corsair vx 550w PSU (41amps on the 12v rail). It has the necessary power connectors (2 x 6 pin) for your GPU (video card) and has plenty of power to handle more things in the future. If you want a little more room for upgrades, I'd go with the Corsair tx 650w PSU (52amps on the 12v rail)
Although I'd buy the PSU's at Buy.com, if your ordering within the U.S.
Corsair VX 550w PSU: $85 - $15 MIR = $70 @ Buy.com
Corsair TX 650w PSU for $3 more @ Buy.com $93 - $20 MIR = $73!!

I'd just buy the TX 650w PSU and call it a day. You'll have plenty of power and have room for more upgrades later. The PSU has a 5 yr. warranty too, so that will come in handy if you ever need to use it.