The minimum amount of RAM required by Exchange is 4G. This is an absolute minimum and is just enough for around 5 mailboxes. Microsoft recommends 6G as the minimum amount of RAM.
For our VPSes you can safely assume that if there is sufficient RAM, the CPU is also powerful enough to allow the server to run without problems.
You should count on adding a gig of RAM for every additional 10 to 15 mailboxes, and if you are using shared calendars and similar features, you should count on an extra gig of RAM for every 5 to 10 mailboxes.
You can find more information about Exchange Server performance here: