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
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).