To start off this guide, I will state that I personally am an ATI fan, I don't know why, I just am. That being said, let's get down to business.
A X1900XTX will outrun a 7900GTX, and a 7950GX2 will outrun a X1900XTX.
These are with the reference clock rates only. At the moment, certain manufacturers (say XFX for instance), are producing stock overclocked 7900GTX's. These cards are no different from the reference ones I mentioned earlier, its just XFX have gotten their hands on some HIGH quality chips, and manually tested them by overclocking them. Those that 'passed' were called the Extreme and xXx editions, those that didn't are what is placed on the 'regular' boards. The same thing has happened to the 7950GX2's. There are no new fancy heatsinks or pipes, and software improvements, or hardware changes. They have flashed the bios to run at a faster clock rates.
Many will ask are the new improvements worth it, and it depends what you are going to do with the card. If you need to be the fastest kid on the block, then yes.... yes they are. If not, save yourself a good stick of RAM in money and buy a standard one. But for those of you planning to overclock a stock card to the Extreme or xXx speeds, then I would recommend buying the faster models, as they come with a warranty for these speeds, AND WARRANTY = GOOD.
So, that being said, a 7900GTX can run faster than a stock X1900XTX. But when you look at the 'TOXIC' X1900XTX with the water cooling and all, Im sure if you tweak its clocks a bit it will be back on top. The water cooling on this card is awesome, I saw one in action and it definately has an impact. Everyone knows how loud ATI cards are (be honest now), and that a 7900GTX will run quieter.
As for SLI, this is a different story. Normally the cards run in as 7900GTX --> X1900XTX --> 7950GX2. This does a complete backflip when in SLI. I am sorry to say, but Crossfire sucks big time, I wouldn't go saying its useless, but for the money you need to fork out to buy a CF Ready motherboard, a CF Ready Card, and then the CF Master card (which IS usually higher in price than the regular cards), the minor gain just doesn't seem worth it.
The new order runs with the 7900GTX SLI out in the lead by maybe 10-15%, depending on who you talk to??? From there on the 7950GX2 and X1900XTX fight it out for second, again depending on who you talk to and the day it was tested on.
To anyone planning to buy a 7950GX2 to Quad-SLI it, forget it! NVIDIA has said that they have not solved the 'quad-SLI problem' yet.
So in the end, a 7900GTX is faster (when OCd), runs quieter, and at a lower temperature, and it damn well costs less.
Hope this has helped at least someone out there...