Yea, well, Tremulous (Or s[ecifically the Q3 Egnine, or Tremulous client) was designed for low end computers that has 512 mb or less, the developers probaly put theat limitation on there for two reasons:
a) They didn't have RAM larger then 512mb back then (As far as I know)
b) Tremulous doesn't need 2Gb (Or whatever) of RAM because theres no mods/whatever that could possibly take up that much RAM (Like what Bissig just said), so why don't you just leave it at 960mb and call it a day, the only Tremulous could take more RAM is either making a compatible client of doing so, changing the engine limitation, changing the cvar limitation (Again, what Bissig just said), etc.

Besides,you don't really need more Ram for Tremulous to run, if you want Tremulous to go faster, go to a server with low ping.
