And technically Snort is a NIDS (Network Intrustion Detection System). It only detects things that come across the wire that look funny.
As far as monitors go, I'd check out
Nagios. Nagios is the new name/version of
Net Saint.
I would also suggest that you look at
Log Sentry from Psionic and
AIDE or
TripWire.
Log Sentry scans your system logs and looks for strange things. AIDE/TripWire keep information about each file (at least the ones you care about) and then can be re-run to insure that they have not been changed.
If you are doing hosting, SNORT is not enough. A valid user could access your server and then use some method to gain unauthorized access (i.e. become root etc.). SNORT will never catch that, because the access the server is a normal manner (i.e. telnet/ssh).
Frank