A question about physical and virtual memory:
In my case I have 1GB physcial memory and 2GB swap space.
After a while, some programs started, the kernel store pages into swap although all the memory requested by the system and all the running programs would be fit within the 1GB physical space.

If I start more and more programs, it is not obvious why the linux system is not using the maximal physical space.

I'm not an kernel expert, I know a little bout memory pages and that programs will use only explicit allocated memory.

Here's a picture from the system monitor:

445+225 = 670MB , that's would fit into 1GB isn't it?

Is that a screen from Linux? It looks like Windows to me, for some reason.
Google "Linux ate my RAM" as I think it will answer your questions.

The screenshow comes from GNOME system monitor, it's really linux.
The question is not why the memory goes away, the question is why the memory pages doesn't gathered onto the physical site, why the kernel work's as this screenshot shows.

An operating system needs to accommodate all potential users of what is after all a general purpose environment. Not just a couple of currently running programs. Have a look at /proc/meminfo for an idea of some of the considerations in play. At all times.
Then try and figure out what that monitor is actually displaying.