I am running IPConfig on a PIII 1ghz with 512mb ram (HP e-pc 40) using CentOS 6.3 with Postfix/dovecot/apache, and it works fine. But what you will see is that linux will use unused RAM for cache, so if you do a free -m it will show you whats being used including cache.
If you take away free, shared, buffers and cached from the total it will give you the amount of RAM actually being used by by the OS and apps. In the below i have about 205MB of RAM used.
[root@tui ~]# free -m
total used free shared buffers cached
Mem: 498 459 39 0 61 193
-/+ buffers/cache: 203 294
Swap: 1022 23 999