In our environment we use one central monitoring nagios server and 2 distributed.
On central server we have around 200 active hosts and 300 active services plus 130 passive hosts and 630 passive services.
Central server is Pentium 4, 2,4Ghz with 512MB RAM. the distrubuted ones run on VMWare ESXi 3.5 (Quadre Core Xeon) in virtual machine with 256MB RAM.
All seems wo work OK and we have not seen any performance issues so far.
Hope this helps.