For some basic monitoring:
Grab the Nagios check_nginx.sh script: http://exchange.nagios.org/directory/Plugins/Web-Servers/nginx/check_nginx-2Esh/details
(Nagios checks are generally scripts that will return specified exit codes for events -- 0 for OK, 1 for WARN, 2 for CRIT; you can use them outside of Nagios.)
Turn on the stub stats module in nginx and have it on http://localhost/stats or something like that: http://wiki.nginx.org/NginxHttpStubStatusModule#Directives
Point the check script at the stats URL: you'll see connections per second. Set a threshold for it with the -w or -c option. Wrap that in a shell script that will send you email if the script returns a non-zero exit code. Put the script into cron.
The page about the Nginx module has some links on the bottom about other monitoring options.
You can install Monit (there's a free, self-supported package for most Linux distros). There's apparently some stuff on Apache monitoring here: