What are you doing when the system crashes?
It is only catalyst or the actual mac?
it it your own, or a hire mac?

can you create a support PDF profile and post it here. (in Catalyst Menu)
can you send me a few of the suspect files

what are you trying to achieve - whats the reason for such a large still?
what are you outputting to and at what resolution?

PNG's aren't usually a problem...

Simon