You have probably already got sar on your computers. It comes with the sysstats package. You just have to set it up. This is what I did on OpenSuSE to get it working.
First I found a file called /etc/sysstat/sysstat.cron. It looks like this.
#crontab for sysstat
#activity reports every 10 minutes everyday
-*/10 * * * * root /usr/lib/sa/sa1
#update reports every 6 hours
0 */6 * * * root /usr/lib/sa/sa2 -A
Then I just created a link to this file in the /etc/cron.d directory.
root> cd /etc/cron.d
root> ln -s /etc/sysstat/sysstat.cron sysstat
This creates reports in the /var/log/sa directory. Then you use the sar command to read the files.
root> ls /var/log/sa
root> sar -R -f /var/log/sa/sa.2006_09_20
Linux 188.8.131.52-0.21-default (myhostname) 09/20/06
00:00:31 frmpg/s bufpg/s campg/s
00:10:01 51.42 0.53 2.28
00:20:01 2.46 0.00 0.06
00:30:01 -1.42 0.00 0.02
00:40:01 -0.12 0.00 0.00
00:50:01 -0.22 0.00 0.00
01:00:01 -0.51 0.00 0.02
You can find out all kinds of system performance metrics by using different options to the sar command. Check out the man page for sar.
If you for some reason (managers) you want to purchase something really expensive then BMC Patrol is very good.