[CentOS] real used RAM!

[israel.garcia at cimex.com.cu]

List, I've installed my CentOS Server running progress 9.1D database
system, with only 15 users working every day, this server has 2GB RAM,
so, my question is: 
Why the 2GB of RAM is always used? Even with only one user connected...
How can I messuare the real RAM MEM used by my system split in proccess?
Is there another tools which I use to compare the results from top. Or
vmstat?

Regards,
Israel