The PSU guide linked in my sig will help some.
There are a number of calculators out there... but here's how I usually do it.
I first discover how much power the graphics card(s) will pull from the PSU
http://forums.atomicmpc.com.au/index.php?showtopic=264
Then I add 200W for the rest of the system... a very generous amount that leaves a little margin.
That gives me a "max draw" from the PSU, so I know that is the minimum.
From there I look at what the normal load will be, whether gaming or whatever. This will be a number smaller than the max load. Then I try to fit a PSU that will be about double that. So that the PSU is operating at a max efficiency... but that is less important and may not be the case if you are trying to buy a PSU that will cover future upgrades.