How big of a PSU for Video editing system?

Well the GPU makes the MOST difference as to what you need, hard to say without that, but a general one card system can use a good 500 watt just fine, better to get a 550 like a corsair just to be safe. If you end up getting a card like a 9800 gx2 (really 2 cards ) then I'd imagine 650 watts at least.

My system runs fine on a 450 just as an example, but if you asked anyone they'd immediately say at least 550, even for me.

Your gpu is really gonna influence your needs more than anything else, then the quad core, the rest doesent use much.

Also, if you are going to overclock, know that power use rises like mad so 750 would not be too much.

A good 500w will be more than enough with room to spare for upgrades.

I recommend the same brandnames as chookman with Seasonic being the top choice since they also manufacture PSUs for the other listed brands.