I agree with sednet; a mailserver itself isn't going to cause a serious load unless you handle truly epic volumes of mail.
Probably the biggest resource hog when it comes to mail is content-based filtering (e.g., Spamassassin). With the recent CPU and memory upgrades, this is less of a concern. But you're best off if you can reject spam
before it gets to Spamassassin; I talk about my setup in a couple of
recent posts. A large proportion of spam can be dumped just with basic sanity checks, greylisting, and, if you like, a DNSBL - all of these require few resources.
Set things up properly, and even with Spamassassin you should be fine, especially on a 2048.