I'm not sure how much RAM will be consumed by WordPress MU per FCGI instance, but we're using the Symfony framework (
http://www.symfony-project.org/) - which is notorious for its bloat - and a PHP opcode cacher (XCache, APC etc.) and our FCGI processes each consume up to 40MB - 60MB of RAM (with about 1/4 - 1/3 in shared memory).
I would assume WordPress MU has been refined enough to be much less bloated than Symfony (although I could be wrong), so would probably consume a lot less RAM.
RAM usage per FCGI process would also depend on the maximum requests allowed per process (the lower the value, the less chance of memory creeping up).
How many FCGI child processes do you have running at any one time? With 360MB of RAM in total, you should probably set this low as well.