OK, time to answer that little misconception.
Firstly, before that year is out the R520 cards will be out and available, and they'll do double the X850XT performance with a single card. In addition to that, don't pin your hopes on the current top of the line card hanging around getting cheaper and cheaper. History proves that just doesn't happen.
When the next generation of display cards comes out, the top of the line cards from the previous generation rather rapidly disappear from the marketplace. A cut-down version of it will linger for a while till the mid-range cards of the next generation become established.
If you want to use two top of the line cards of a particular generation in SLI configuration, you gotta get them them from the outset or shortly after. That's for NVidia SLI.
ATi Crossfire is a little different. That requires a new card which can act as a 'master' card. So you're gonna be able to get that X850XT Crossfire master and a new Crossfire enabled motherboard and do the job. But hey, all you're gonna do is get the same performance as you would if you waited and installed an R520 based ATi card anyway, and it's most likely gonna cost you just as much!
Sorry, but I don't see much point in that