total used free shared buffers cached
Mem: 992 844 148 0 92 361
-/+ buffers/cache: 390 602
Swap: 2015 0 2015
is my output from a pretty much fresh install on CentOS 6.4 with dovecot, squirrel mail, nginx etc. (perfect server tut)
Perhaps you could detail what sort of install you did (e.g. any predefined template or extra packages, or just a minimal OS install?)
If RAM is an issue, consider switching over to nginx, that'll help a bit.
Also, combination of clamav and amavis tends to take up a few 100mb of RAM - perhaps consider an alternative to them maybe.