What resolution is your composition? AE CS3 crashes all over the place with multiprocessing and more than 6 processors if the composition resolution is above 2k. See this article:
http://kb.adobe.com/selfservice/viewContent.do?externalId=kb402403&sliceId=...
Depending on how often you work 2k or above, it might be worth following the instructions to restrict the number of render processes to 6.
I normally work in 1280x720, so I haven't done this and don't normally have a problem, but I run in to trouble and have to disable multiprocessing when I occasionally do have to work with something larger.... it sucks and I hope they fix this soon.
-Matt
