Carrara does have a virtual memory set up for the memory spooling of textures. As a rule, real RAM is way better than spooling. However, if that is not an option, you'll get the best results by setting 100M of spooling (virtual RAM) per Gig of RAM. It will boost save times - but not as much as real RAM.

Using more or less than that is up to you and your system, so you can experiement. However, when I talked to one of the developers, that was the rule-of-thumb he gave me.