The "recommended power supply" with graphics cards is somewhat accurate, somewhat misleading. When they make that estimate, they are assuming some basic computer setup, which may or may not match yours. You seem to have a fairly basic setup, so that recommendation is probably fairly accurate.
The best place to go these days for a good estimate is PSU Calculator - Take that value and give yourself some headroom when looking for a new power supply.
GTX 260's draw about 150W under full load, and depending on your processor, it can draw that much or more. (Not sure what the
^2 is, are your running in SLI?) 30W for motherboard + 130W (core i7, just using for estimate) for processor + 150*2 for SLI GTX 260's = 460W, and that doesn't include USB periphrials / harddrive / ram / optical drive.
The card won't run slow if you overload your PSU - The system will simply freeze, blue-screen (or your OS's equivalent), or crash. And it may work fine until you load up a particularly intensive game. It may run for most games and only show problems if you stress-test the system. This shouldn't damage anything outright (nothing will explode in a flaming ball of fire), but it's never "good" to run components on the edge like that, and you do risk the contents of your harddrive when an unexpected shutdown occurs (less these days than in the past though)
To simplify, grab yourself some Crysis or Supreme Commander and see if your system freezes or crashes. If it does, or if you want to make adding a harddrive a worry-free process in the future, go ahead and pick up that 600W power supply. (Remember, the power supply rating is the max, not how much your system will draw, so you're not going to waste power by getting a power supply that's bigger than you need)