You look at the minimum required for your video card.
and multiply it by like uhm.
3.
4 if you want to be safe
=)
Or at least that's what I do, LOL :/
But really, just check out the minimum for your video card.
Then check out the min if you were planning on doing SLI.
Then get at least that...
Or go with like at least another 100w to be safe.
Depends on your budget ;p