Yes, your PSU is strong enough. As for gts vs gtx, it's hard to justify spending over $500 on a GPU that's been out for almost a year. Regardless, the gtx is a good bit faster, and if you're gaming at high resolutions you don't really have any other options.
Speaking of which, maybe you could get a HD2900Pro when it's available. It does a bit better than the 8800 GTS 320 in some games, a bit weaker in others, consumes just a bit more power (still fine with 520HX). I've just read a review about it and I think I like it. They say it will sell for about $250. The only thing I don't like about it is that it makes 10 dB more noise than the GTS, but that may be OK in a good case.
Don't believe the conclusions, the HD2900Pro is NOT as fabulous as they conclude, if you read all the pages. Still, in that price range, it's very hard to say if this or the 8800 GTS 320 is the better deal.