Originally Posted by Itzuke
As you can see, the System Guard says that about 240 of them are being used...
Not really. You're using just under 100MB of RAM the rest (yellow) is cached and will be discarded when some process needs to allocate memory. Linux will always use almost the entire physical memory to store cached data no matter how much RAM you have:
demian@dirac ~$ free
total used free shared buffers cached
Mem: 1034664 1020748 13916 0 16272 612320
-/+ buffers/cache: 392156 642508
the last line gives you a better idea of the amount of memory that's available.
If you still want to check what processes use the most amount of memory run top and sort it by memory usage (M).