Well the X1900 XT is a 512mb card and the 7900gt is a 256mb card, both great cards for playing your games with max settings @ great frame rates. Basicly the only difference is the fact that you can run your textures on games a lot higher with better frame rates with that 512mb card, and the 7900gt will run your textures more than fine on very high settings with more frames than the human eye can see.
Both ATI & NVIDIA are great in terms of providing us with great gaming proccessing power. And of course the more money you spend on the card doesnt mean your buying that much more power. You can be spending almost 400 dallors on that ATI card for maybe who knows, 10-15 more frames on insanely high textures even though the human eye only detects something like 60 frames per second and just be more than okay with that 7900gt 256mb on all max setting and your eyes wont even notice the difference.
The verdict is that the 7900gt is the ideal card for gaming over that x1900xt if you dont want to waste a extra 100 dallors for more frames that your eye cannot even notice or if u want to have your textures maxed out completlyand run fine. Though the ATI card is better performance wise. I mean dont get me wrong that x1900xt (ATI) has some higher benchmarks in different areas because the core clock is @ 625MHz than that 7900gt (EVGA)which is @ 580MHz, but your spending more money than what you should for little performance increase. If you have the money of course go for the 512mb card. Keep in mind NVIDIA has better benchmarks with games that run heavily on pixel shader as ATI sometimes does not.
My advice go for the 7900gt you will be playing your games with maxed settings at great resolutions with at least 60 fps and higher. Also check out benchmarks on websites and all that jazz, thats more proof on which is better. Research and find out for yourself.