I need to monitor the memory & CPU usage on a LAMP turnkey server. Any recommendations on a good solution to monitor these resources (and maybe more)?
I highly recommend using collectl. I've spent a pretty good amount of time in the past looking for what I think is a good solution for this type of thing and collectl continues to come to the top of my list. What I like the most is that you can capture a lot of different statistics and view it all in one tool at command line. This is far better than using multiple tools and trying to correlate the information from each tool. It can also produce output that is ready to be used as gnuplot input so you can generate graphs. It has great granularity for troubleshooting performance related issues.
If you just want simple graphs of a small number of servers you can install munin. munin master collects data from munin nodes. A munin node must also be installed on the server running munin master if you want to collect stats for that server. So if want graphed stats for a single server then install both using the following command.
apt-get install munin munin-node
Then just configure both munin master and node for your purposes.
If you want to do more advanced statistic capture and graphing I definitely recommend cacti.
More information about formatting options