Hi...
I'm running zimbra on a Tiny widdle Asus Terminator P4 2.4 GHz with (a shameful) 727 Mb Ram, Running on Ubuntu 6.06
I only have about 15 Users (friends and family) who use it quite frequently, on that same machine, I am running an apache web (with php and APC php cache) server MySQL Squid Proxy Postgres SQL, FTP and BIND DNS, and that little machine runs perfectly, I never have ANY problems, especially since I moved to Ubuntu (Used to run FC6).
So, if your hardware aint that grate, and you lookin for a stable machine, I would reccomend you go for Ubuntu, Its the most stable linux distro I have ever come accross and it has a rock solid kernel.
"JUST MY OPINION

"