The reason you go with dual 6800GTs or Ultras instead of a single x850XT PE is that the dual Ultras will last longer in terms of performance.
In Doom3 on that X850XT PE you could play it with the visuals maxxed @ 1600x1200 w/ 8xaa/16xaf and still get around 80fps. Not bad huh? I sure wouldn't mind it.
With the dual GTs, or Ultras (referring to the 6800s, NOT the 6600s), at the same settings you get anywhere from 160 to 200fps.
Ok, that 160 to 200fps wont be noticable to the human eye, there for you don't get it for that. If you are only getting 80fps on that Radeon, those fps will continue to drop as the games released continue to have increased visuals.
If he were to get that Radeon, he'd enjoy 80fps in today's current games with the visuals maxxed. BUT... by this time next year, he might be getting only 60fps on THOSE current games. Still not bad at all.
Then again on the SLi with dual 6800GTs or Ultras, he'd be getting 120 to 160fps by this time next year.
Ok, now add another year.
That Radeon now only gets 40fps and maxxed visuals, the dual 6800GTs or Ultras still gets around 100fps.
Hmm... you could notice the difference I'd imagine.
Add another year, that Radeon with maxed visuals slugs along at a mere 20fps while those dual 6800GTs or Ultras could be getting him an honest 40 to 60fps still.
Now, it is obvious that my numbers are going to be off, I'm not a prophet. But... the concept is accurate. As time goes on the games will come out with better and better graphics. As such, the vcard(s) will get stressed more and more as the graphics continue to increase. Thus, the frame rates will slowly drop. That 80fps wont last forever, just like that 160 to 200 fps on the dual 6800GTs or Ultras wont last forever, but the high frame rates will last longer on the dual 6800GTs or Ultras than the single Radeon.
Keep in mind- this goes for both ATI and nVidia as both offer an SLi solution, just nVidia's is already on the market and has been for some time.
Games will be comming out with SLi in mind, for BOTH nVidia and ATI. But untill ATI has thier SLi in the Market and on shelves, those games will be aimed at nVidia. We don't see SLi's full potential becuase current games were not made with SLi in mind. They were made with single card rigs in mind. That will slowly change. As time wears on more and more games will be made with SLi support, thus game devs will fully exploit its potential- just not yet.
With that said, one X850XT PE will offer approx. half the performance of dual 6800Ultras when each card is compared when visuals and res. are both maxed.
So, ATI's SLi might offer more than nVidia's- but nobody knows yet. Only time will tell. You may get a rig with dual Ultras just to find out that a month later ATI's SLi is out and offers a 30 to 50% performance increase OVER your dual Ultras. Boy would that suck.
Then again, by that time nVidia will be close to releasing the GF7. That gpu could be a BEAST. Nobody knows how the NV50 series of gpus will stack up compared to the NV40s nor the R520s or R6xx. Only time will tell. The NV50s will be natively PCI-e, that is known. They will fully utilize that extra bandwidth dwarfing AGP cards in tems of bandwidth. They will also be made with SLi as the main focus (the high end cards will be, not so much the low end- not yet anyways). As such, it will be safe to say that dual NV5x's will SPANK dual X850XT PEs- or atleast they should. The GF7 might end up another GFFX for all we know.
If I were to be in this situation I'd look at this:
nVidia already has SLi out and is in the process of perfecting it. ATI does not have SLi out and will have to perfect it over time. As such, nVidia- theoretically- has the advantage since they got the head start. I would then think that if I got dual 6800GTs or Ultras now, I might be surpassed by the dual Radeons for a short while. But when the GF7 is released, I could very well retake the lead leaving the Radeons behind yet again.
After that- both nVidia and ATI may become "tied" (referring to after the GF7 and Radeon XX00 series). Those cards are two generations away and by that time ATI may once again be in the lead.
But, for now it is clear that nVidia has the performance advantage, and their drivers ARE less buggy.
Get either one 6800 Ultra or 2 6600GTs and SLI them.
Yes, but then no. Okay, you say 2 6600GTs and SLi them? Why? One 6800GT can more than double that performance for the same price, if not less.
Okay, 2x 6600GTs is about $400. One 6800GT is only $350 to $400. So, you could say that the price for 2x6600GTs is roughly the same as that for ONE 6800GT. Also, that ONE 6800GT provides the same performance as 2x6600GT- with the cards at stock settings. Now OC that 6800GT to say 500clock/ 1300mem. You have now paid only $400 but are going to get MUCH more performance than the dual 6600GTs could ever offer- even if they were oc'd. The 6600GTs just don't pack the performance punch of the 6800GT. That is why 2x6600GT= 1x6800GT. Now, I say get the one 6800GT becuase it is on average $150 to $200 cheaper than the Ultra, but can offer JUST as much performance with a little OC.
Definitely 6800GT or Ultra if you go SLi. If you go with Dual Ultras, the only reason should be for the 512mb version as a oc'd 6800GT can provide just as many fps as the 256mb Ultras.