Wattage doesn't matter as much as how many amps are on the +12v rail. There should be a requirement for that somewhere for the specs of the video card. Pretty sure there is a sticker on your PSU that shows the number of amps on +12v.

I would recommend a new PSU if you decide to buy a 8800gt.

I wouldn't get the 8800gt though, I would add more RAM to your computer.

You cannot SLI different series of cards, they must be exactly the same.