It locks up indefinitely. But never crashes. Sometimes it recovers long enough to click on another image, only to have it lock up again. When it's frozen I can't do anything else on the computer. The only way I can get the system to work again is wait long enough to quit Aperture, or SSH in and kill the process.
I just swapped memory from my MacBook into the IMac, so it now has 2 x 1GB chips. For a lttle while it seemed to be working better, but then froze again. So far, I've only noticed this problem when edit RAW files, but not with JPEGs or largeish (50mb) TIFFs.
The only other programs running were iTunes and iChat. I turned them off, and it still froze.