I'd say you're probably safe with a 400W unless you have a quad in which case a 450W might be better. If it's a good PSU you'd prolly be fine with a 350W. You might consider upgrading to one of these:

http://www.newegg.com/Product/Product.aspx?Item=N8...

It's about 2x as fast and probably consumer less power.

I doubt any new PC (unless it's a super-cheapy) would be unable to power the 7900gTx though. Look at OCZ power supplies on newegg.com if you need to replace your current one. They're usually a great deal.

600W is totally outrageous. I have a Phenom 9850 (125W quad core) and 2 ATI Radeon HD 4850's on a 550W Antec NeoHe power supply. No problems. The 7900gtx has a peak consumption of 85W. My radeons are 120W a piece and I have two. You should be totally fine with a 400W, or again, probably whatever PSU you have.

There isn't a specific answer to this because it depends on the draw of power from the rest of the computer. Additionally it depends on the efficiency of the power supply you are putting in.

I would suggest around 600w in general, but make sure it has the connectors you need based on your old power supply and the new video card you are adding. Additionally, make sure it will fit in your case.

Here is a good bit of information on them:

http://www.xoxide.com/buy-computer-power-supplies....