SLI 7900GT will beat all the ATI cards in a single card configuration, However for the cost, I would definately go with a single X1900XT since it performs so closely to either of the 2 choices you asked about.
A single X1900XT (Not XTX) is more than enough for overkill on games for the next year or so, at least until the next-gen of DX10 gfx cards come out. Sure, a 7900GT SLI config would be awesome, but it will cost you a lot more and the benefits won't be near enough to balance out the cost, plus you need to shell out on a reliable high-end SLI mobo, such as DFI, in order to make sure everything runs nicely.
Well personally I just built a new rig and put a x1900xt in it. I wouldn't go with the two card solution from the standpoint that they don't have all the bugs out of sli plus it isn't supported well on very many games. Go with the ati. My opinion of course but I converted on this one from a 6800. Only way to be informed is to try them both for yourself I guess
A single 7900GT can provide very nice framerates in all titles at most reasonable resolutions...
If determined to play FEAR at 1600x1200, and a hardcore advocate of 8xAA/16xAF, either solution (1900XT/XTX or 7900SLI) provides a very nice jump in framerates, with a pair of SLI'd 7900GT's usually outperforming even a Crossfire rig, much less a single XT or XTX.
You personally have to decide wether the added expense of a 1900XT/XTX or 2nd 7900 card (and an SLI MB, if you don't already have one)is worthy of an extra $200-$350 over the cost of a 7900GT...
I personally feel a single 7900GT is at a truly great price/performance point ($299), especially for those limited to 1280x1024 and below by virtue of owning 19" LCD monitors, followed by the impressive 1900XT or 7900GTX ($499) if one is willing to fork out that kind of money for gaming video at all, and then followed by SLI 7900GTx2 ($599)! Each price point allows a progressively higher resolution and/or level of AA/AF settings, or both. (Id not consider a >$600 solution unless you have at least 1600x1200 capable monitor, or 1920x1080 widescreen)