Um in Nagios Infos via SNMP abzuholen habe ich dieses unter Debian wie folgt eingerichtet.

apt-get update && apt-get install snmpd
nano /etc/snmp/snmpd.conf
rocommunity public  localhost
sysContact     Manuel Blickle <mail@blix0r.de>

/etc/init.d/snmpd restart
snmpwalk -v2c -c public localhost 1.3.6.1.4.1.2021.4.6.0

# Configure authenticated snmpv3
/etc/init.d/snmpd stop
net-snmp-config --create-snmpv3-user -ro
/etc/init.d/snmpd start
snmpwalk -v 3 -n '' -l authPriv -u "User" -A "password" -X "password" localhost 1.3.6.1.4.1.2021.4.6.0
# Output example
#> iso.3.6.1.4.1.2021.4.6.0 = INTEGER: 105544

cd /usr/lib/nagios/plugins
./check_snmp_mem.pl -H localhost -C public -w 80,79 -c 90,89 -N -f
# Output example
#> Ram : 53%, Swap : 18% : ; OK | ram_used=535908;813837;915566;0;1017296 swap_used=93940;414184;466613;0;524284

Quelle:
Howto: SNMP(D) unter Debian Squeeze
Configure authenticated snmpv3 service using net-snmp
check_snmp_storage.pl , check_snmp_load.pl