Memory question

Sorry for posting an obvious Linux question, but have any of you
encountered this and how have you fixed it.
I have 6gig Ram box. I've set my shmmax to 3072000000. The database
starts up fine without any issues. As soon as a query is ran
or a FTP process to the server is done, the used memory shoots up and
appears to never be released.
My fear is that this may cause problems for my database if this number
continues to grow. Below is my TOP after running a query, and shutting
down PgAdmin. While not low now, the amount of free memory has dropped to
around 11mg. I'll admit I'm not that Linux savvy, but am I reading this
correct?

--TOP

45 processes: 44 sleeping, 1 running, 0 zombie, 0 stopped
CPU0 states: 0.0% user, 0.0% system, 0.0% nice, 100.0% idle
CPU1 states: 0.0% user, 0.0% system, 0.0% nice, 100.0% idle
CPU2 states: 0.0% user, 0.0% system, 0.0% nice, 100.0% idle
CPU3 states: 0.0% user, 0.0% system, 0.0% nice, 100.0% idle
Mem: 6711564K av, 6517776K used, 193788K free, 0K shrd, 25168K
buff
Swap: 2044056K av, 0K used, 2044056K free 6257620K
cached

Patrick Hatcher