The ram/system memory has little to do with whether you can run the card or not, 2GB is more than enough, though for running Vista, you might want to bump that to 3GB or maybe even 4 if you can.
as to your PSU, yes You have 2 6 pin PCI Express connectors, one on your current 7900GS, & one spare one, if you get the 9800, you'll need to connect both, but for the 9600, only one is needed.
the Page file you listed, is the amount of Hard drive space being used for temporary system memory... the more RAM you have the smaller the page file generally is (actual amount used)... you can specify the max amount to use too, with in windows, but many generally like to leave it up to windows to decided the max amount of drive space to use.
and that link I posted on the first or 2nd page of this thread, the same exact one john just linked to above, is one of the best deals online for a 9800GTX right now, plus even better yet, its an eVGA brand, one of the best on the market, & also you should be able to enroll in their step up program, which basically allows you to trade in your "old" card & move up to a newer one, & just pay the difference in price between them... so, say you bought a new 9800GTX for $300 now, & in the fall they came out with their new series card, thats a good bunch more powerful, for say $350... more or less you pay $50 more & trade in old card, & you get a new one...
its a bit more than that, but thats the jist of it...
http://www.evga.com/stepup/default.asp