Most distros come with the top utility that can give you all sorts of stats about the current system (load, memory usage, I/O wait, ect.).
A lot of things in Linux (especially in X) load a lot of "stuff" into memory. This alone doesn't cause a problem, unless you are also loadin big stuff into memory. GNOME and KDE (the prettiest window managers) use the most amount of memory, if the memory starts to cause the system to slow down, you can take two options:
First, you can turn animations down or off (in KDE at least), and uninstall windowing themes that you aren't using.
Second, (more drastic) you may consider using a smaller window manager like flux, fvwm, twm, elightenment or some of the others out there. What you might be giving up (depending on which manager) is stuff like:
- multipule desktops
- rounded corners and window themes
- pretty animations when things are clicked.
- easy to use gui to set up color scheme (meaning changing colors/backgrounds through configuration files)
Just some suggestions.
[plug=shameless]
[/plug]