Ok we have a lot of fundamental errors here:
People keep bringing up how little SLI does. You seem to forget the small little tidbit about not every game supporting SLI. Nvidia must create a profile for SLI to be supported. Obviously if you lose or gain only 1 or 2 fps then SLI is not working for that game (The variation is well within the margin of error).
Next, drivers. All you hardcore ATI fans out there neglect the fact that their Linux drivers are NO MATCH for Nvidias. I do like how ATI releases new official drivers every month and i wish Nvidia would. Zakee you mention a slew of problems with HL2. So far i know of 2; both of which are fixed. The shiny grass is rumored to be fixed in the 71.84 revision (see drivers thread) and yellow water is supposedly fixed in 75.90. I dont know what other errors you are seeing.
Next, you say that ATI wins in overall performance or brute force. While this is true think about this. The MAX difference you will see is in HL2 and is ~17fps with settings maxed, however you are already in the playable range. All the other games are very close, with the X850XTPE winning in most of them by a small margin. Why trade cards for a 5% increase.
Features. Nvidia fans are saying they have a PVP SM3.0 32bit FP Precision, ATI fans are saying they have some Video Acceleration (through the pixel pipeline) and 3dc. The fact is that neither sides features are really used; however in the long run Nvidias features seem more useful. Not many games are planned to use 3dc, games in the future will use SM3.0 IIRC 32bit FP Precision is becoming mandatory for later version of DX. While the PVP isn't "working" in its entirety ATI's cards do almost the same exact thing only they use their exceptionally well refined Pixel Pipeline to do the hardware encode/decode.
This all boils down to how long are you keeping the card.If you plan to upgrade first thing next gen, then you might as well get the X850XTPE. However if you plan to wait a while the 6800U offers more than just brute force.
To address price... this is really a moot point. The Nvidia cards are available in much greater quantities at much closer to MSRP. Understandably so as ATI has also released a "fall" refresh whereas Nvidia has not so they have a lot more to cope with. You would be hard pressed to find a high end ATI card close to MSRP unless you got in on a lucky deal or sale.
I do notice a lot of people in this thread have no basis for their opinion and simply come in here to cause trouble:
Perfect example :
rollo = pwned
Stop trying to push SLi, your comparisons are only of those that ARE close. How about the other games where the x850xt pe kills the ultra?
end yourself.
Instead of just speaking the first thing that comes to your mind why dont you back up your opinions with some good evidence or at least some technological know-how.
Having said all this if you plan on upgrading immediately after the next gen cards are released then go ahead and go for the X850 however you might lose some features along the way. If you plan on sitting a while and not upgrading first thing the 6800U has a nicer feature set (ie more future proof as compared to the X850) but is a little slower.
-Kevin