Redhat Memory Question

I have never monitored any Redhat systems before, all Solaris in the past for the most part. Since top and free always show all the physical memory in use on Redhat, how do people monitor memory usage accurately with Xymon? Currently the Memory tag shows as yellow under this new Redhat server, though in my mind it should be green.

Mem: 16435916k total, 16311612k used, 124304k free, 6255112k buffers Swap: 18481144k total, 0k used, 18481144k free, 6391000k cached

         total       used       free     shared    buffers     cached

Mem: 16435916 16311588 124328 0 6255116 6391040 -/+ buffers/cache: 3665432 12770484 Swap: 18481144 0 18481144

Thanks in advance

Wes Neal