The 9800GX2 will have higher maximum FPS, but the GTX260 with its greater amount of RAM (512MB 256-bit vs 896MB 448-bit) will have much higher minimum and average FPS, run smoother, no stutter, and will still perform the same in games that don't utilize SLI whereas the 9800GX2 will perform just the same as her current GTS250 if the game doesn't support SLI. The GTX260 will also allow more AA and perform better at higher resolutions.
I have had G92 SLI, single GTX260, and SLI GTX260's, and I would most definitely take the single 260 over G92 SLI.