I'd go with the GT, but that's just me. If you have money to toss around, and it's not a big deal, the GTX has 4 extra pipes that the GT doesn't. If you're tight on cash, the GT will do just fine.
If you're going to try the 'buy and wait' strategy, I would do so with the 6800GS. It's a heck of a lot cheaper and still runs STRONG in today's games.
PS I will run Windows 64bit pro... on amd 64bit CPU...
There only one card that say that it will run on 64bit OS.. Is it true that one that card will run in Wn. 64... or other will as well?
I think you mean the 7800GT...well anyway the 7800GTX is only about 10-15% faster than the 7800GT. You can easily OC and eVGA 7800GT to GTX #'s for a cost saving around $150. If you want it to last for another year go for the GTX. If you want something to hold you over till the G80, get the 7800GT and trade it into eVGA for the same price of purchase and upgrade it. There are tons of benchmarks out there. You can also SLI two of the same cards together for respectable performance gains. Just to mess with you more...check out a PPU or a physics card.
As others have said, the 7800GTX is faster than the 7800GT by only a FPS and about 10% in performance. But you can find a 7800GT for cheaper than $300, and a 7800GTX's start around $450. You do the math.