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.
1920x1080 makes full use of SLI, and trust me - two GTX 260's rival the very top cards on the market today and can handle every game I've tried so far. Crysis, Hot Pursuit, COD, MOH.. all of them - forget about it.. just crack to highest settings and enjoy gorgeous gameplay.
Even a single GTX570 is quite a chunk of change, but, if playing at 1080P or above, might extend the life of the current PC for another year or even two, depending on the effect of the Rage-engined games.