Since they don't hold mail very long is there any reason if using two MTAs (around 60 users) would need MTA's living on vmdks larger than 16GB?

Currently the system (Ubuntu 10.04) and ZCS mta and proxy are using about 3GB on each system.

I'll have multiple - and larger - disks on my mailbox servers (also going with two here)

...follow up...
How about speed? Can these live on lower IO storage?