The HD 2900xt is a DX10 card. In fact, it is actually the first card designed from scratch specifically for dx10, unlike the 8800. Also it's logical to take into account the fact that AMD has been working one on one with microsoft in order to design hardware to take full advantage of Vista, and ATI started pushing back the release of the R600 around the time that AMD announced it's acquisition of ATI.
Vista has been plauged with driver problems, especially the 64bit version, which i suspect factors in to them deciding to release a 32bit version after claiming for quite awhile that Vista would be exsclusivly 64bit. But there are no DX10 games out, there won't be for another several months so right now performance comparison doesn't mean a hell of alot.
The 8800 is a beast of a card, no doubt. If someone wanted to know the best card available now, for today's games i'd have to go with the gtx ultra. But the card was designed as a dx9 card with dx10 support, not as a full DX10 card, which is what the HD2000 family is.
The 8800 was plauged with driver issues when it first came out, and there were recalls on some, and hardware revisions done to fix some major issues (yes, i did have an 8800 right after they came out, i had nothing but problems with it and it was like a space heater so i returned it and bought 2 x1950s for CF). But as has been mentioned, and was pointed out in the THG article, the HD2900 is aimed at competing against the 320 GTS, yet is comparing with the gtx. Also it should be noted that the 2900 actually has a lower power consumption under load compared to the 8800gtx ultra, it's a gaming card...so that's somewhat important.
The actual hardware design, and stock features on the r600 cards, blow nvidia away. But again, there isn't anything available yet to take advantage of all the cards features. These benchmarks, really don't mean anything, and they won't until the 2900 is compared with the 8800 using the same, motherboard, cpu, ram hard drives and PSU's while running on 64bit Vista ultimate with fully compatible DX10 games.
Right now all this comparing is about the same as Intel saying the Conroe is 50% faster then anything AMD has out. Which is true...if you're using a 32bit OS. If you compare them in a 64bit OS intels lead drops to 5-10% over the 939 stuff that was largely unchanged for a couple years. It doesn't really matter who's hardware is the fastest when using near obsolete software. Which is going to be the case very soon since microsoft finally, formally announced that they will no longer be producing or supporting 32bit OS's or software come 2008.
This doesn't matter as much as everyone seems to think. So as far as the 2900:
There are obvious driver issues with the card, and no software to actually make use of it.
No one knows how the 8800 will shape up in DX10 either, and that is ALL THAT MATTERS.
The hardware design is better, anyone that has a decent understanding of hardware should grasp that. Think of trying to run a PS2 emulator on your PC, any PC made with in the past 4 years is easily better then the PS2, but it doesn't mean it will run a PS2 game better, because the game wasn't designed to.
The 1gig r650 or r680 whatever it is, will be coming out in a few months, which is really the test between Nvidia and ati. When the 512 ati card is comparing with the 768 gtx ultra in some things i think it's worth waiting for it to come out especially since by that time there may actually be some DX10 games available.
If you feel you just have to get a new vid card today, If you want the fastest thing around right now and have $700 to spare get the 8800gtx ultra, but be prepared for the possibility of regretting it in a few months. If you want something with longevity that performs quite well now, is DX10 compatible and will be useful even after you've replaced it as your top end gaming card go with the $400 HD2900.
If you have a card that you paid $400 for in the last 6 months, just wait it out. I can run everything in 1600x1200 with everything maxed including oblivion, most things in 2048x1536 with everything maxed and numerous things in 2560x1920 with AA on 2x using only one x1950 card...not even a concern running in CF, so there isn't much out really regarding current game titles to expeirence that much discomfort unless the rest of your comp sucks.