Apparently in the Q3 engine it tells how much memory you want to let the game use. The more memory you let it use, the better the game should run (assuming you actually HAVE that kind of memory to spare!).
Typically I think it was supposed to be one half of your total system memory (available RAM) for best results.
But the thing is, when Jedi Academy came around, Raven developers SPECIFICALLY said that the memory is "auto assigned" based on your system specs the game auto detects. So setting it to 1000 will do nothing better than setting it to some other random number (in JA.. now JK2 is another story!). They complained that people STILL insist that they get better performance when they set certain numbers, when in reality it's all in their imagination. The game just ignores whatever you put in, according to them.
When it did do something, you set it in the cfg file that you run with the game, not in the command line.