Debian Memory Requirements
Each Debian release and architecture has its own (minimum) RAM requirements, which mainly depends of the type of installation (Graphical Desktop...).
GNOME users can use the About section in the Settings to know the installed RAM.
free is the Unix standard command to display the amount of free and used memory in the system. For example:
$ free total used free shared buffers cached Mem: 2067696 679244 1388452 0 111500 123236 -/+ buffers/cache: 444508 1623188 Swap: 998572 368976 629596
RamDisk - use RAM as a disk partition
NotEnoughRAM - using Debian on low memory systems
Swap - swap space (virtual memory) management