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)