I disagree strongly with Glen M - the amount of memory available for cache and buffers is essential to ensuring good performance on nearly every type of application on nearly every type of server OS.
If you wait until your webserver starts paging before you know about the problem then it's much too late.
On a handy workstation, I can see this:
total used free shared buffers cached
Mem: 4135700 2588568 1547132 0 91024 2309052
-/+ buffers/cache: 188492 3947208
Swap: 4192944 156 4192788
I'd want to report usage as 100*[used-buffers/cache]/total, or 100*188492/4135700.
Writing a plugin for this is not exactly taxing. But there are several offerings available which cover this - e.g. check_mem.pl
To build your own, here's a wee hint:
PCTUSED=`free | awk '/^Mem:/ {tot=$2; } /buffers/ { used=$3; } END { print used*100/tot }'`