Well it is actually 3gb on XP and only on 32bit CPUs less than 2-3 years old(they can
process some 64bit commands).
For older CPUs you need to edit your boot.cfg too even get the system to see 3gb.
But then each program will only use a maximum of 2gb
Although it does help to have spare mem to run everything else while you play the game. Unless you really under RAM there is really no point to go buy more if your machine is slow for games, if you have 1gb yeah go buy more but if you already at 2gb, adding more is not going to make that much of a difference for games.
Playing games that require a lot of resources also require a strong machine, and BtS
does require a lot of resources for big maps.
Crevex:
You might need to edit your startup using msconfig to eliminate all the background programs you dont need, disable your internet/LAN connection, disable your antivirus/firewall and lower all the settings within BtS to the lowest possible configuration.
Then depending on your PC specification (which you haven't told us yet) you might be
able to play large maps to some degree.