I guess you are new to linux...
"high" memory usage is normal. http://gentoo-wiki.com/FAQ_Linux_Memory_Management
The kernel caches all i/o transactions. This will fill memory up real quick. However the cached data in memory can be cleared out real quick if an application needs it.
The command to see actual memory usage is
This will show the memory used for apps (buffer) and the expendable memory used by i/o caching (cache)