If you got a quad core you might see between 5-15FPS increase in games depending on if they make use of the extra cores (as not all games or applications do). If you got a second GTX 260 you're FPS could gain almost 95%.
I have two GTX 260 (216)s in SLI myself, and if I disable/enable SLI the difference is night and day. Playing Crysis at 1920x1080, Ultra settings (modded .cfg) I get between 10-25FPS with one card, with the second card in SLI I get between 25-45FPS.
Only issue you might get is that if you get a second card, your CPU would bottleneck them some-what, which means you either need to overclock your CPU a good chunk (above 3.5 is what I would suggest) or get a quad anyway.