If you go to the website of the company that made your video card (remember, nvidia only makes the chips that power it, they don't make the actual cards) you should be able to find its power consumption information. If you can't find it, try emailing the tech support people for the information. Even if they can provide you with a rough range, as long as it's within 5-10W, it should be plenty.
If I had to guess, I would say a card like that probably uses around 150W at peak draw. If you had two of them, that would be around 300W. Assume around 20W for every hard drive and CD/DVD-ROM. Most PCI cards have negligible power requirements, so I wouldn't worry much about those. So then you just need to figure out what your CPU will use, which again, should be able to be found on AMD or Intel's website.
The other thing you'll want to be concerned with, is cooling your system with two high end video cards pumping out heat. You might want to invest in a water cooling unit. Convection cooling probably just won't be up to the task for what you have in mind.
I want to build a SLI machine with two 8800 GTS, but I don't know the combined wattage required. I already have one 8800 gts running. How much power would I need for the second, (I have a Silencer 610 watt PSU). Or alternatively, how do I track my power usage? Thanks...