THA'S HORRIBLE ARES!!! That's also extremely strange. I have the same thing as you (revA iMac with 64 megs built in) and it runs perfectly. Maybe you should take a few plugs and/or recordings out. Or maybe try redownloading... well that's a last resort unless you've got a really fast modem.
I have no idea why it's like that... but I've found that having too many plugs in the plug folder and too many recordings in your recording folder adds onto the RAM requirement, so I think that may do the trick. hopefully.