Ganglia is aimed at monitoring compute grids, i.e. a bunch of servers working on the same task to achieve a common goal - such as a cluster of web servers.
Nagios is aimed at monitoring anything and everything - servers, services on servers, switches, network bandwidth via SNMP etc etc. Nagios will send alerts based on set criteria (ie, you can set it to send yourself an email or if x service dies).
Note that they are not competing products, they are aimed at different scenarios. By the sounds of it, you need Nagios.
If you have a play around with some online demos, you should be able to get a feel for what product you need (and I think you'll agree with me that Nagios is more suited)
Nagios - https://en.wikipedia.org/wiki/Nagios (Wikipedia)
Ganglia - https://en.wikipedia.org/wiki/Ganglia_(software) (Wikipedia)