I think it's right, actually, because of the difference between the way each works. nVidia often has a philosophy (heard from their chief researcher when he visited my university) that if they can shortcut things, and accomplish the same result, then that's the way to do it. Things like not doing calculations for what it looks like behind that tree in the game you're playing. It gives a performance boost to do edge detection, because even though you're processing to find the edge, it's still less than shading everything behind the tree.
ATI may be doing that now, too, but they don't talk about it. Anybody know?
But anyway, it's the same kind of philosophy. nVidia probably isn't going to bother making SLI work on DIABLO, or Bookworm Deluxe, because people are never going to see those games in a context where they need SLI.
Crossfire is mature these days, IMHO. The X1950 series made that so, with the addition of the internal cables.
Finally, if you're going to crossfire HD2600XT's GET THE DDR4 VERSION!
I can't stress that enough. RAM is a major bottleneck for ATI cards atm.
Doing that, though, may push the price above the 8800GTS for a crossfire setup...and I'd say, get the 8800GTS if that's the case.