XP like any 32 bit operating system has a 4GB addressable memory limit. Windows NT and all its variants use 2GB for the operating system and 2GB for user programs. There is a slight tweak to this in some variants (/3gb switch), and some hardware extensions will let you address more, but in general, on a home PC, 2GB is your lot.

this upgrading is addictive, i just upgraded and installed another gig of ram, a new 8600GT graphics card and a Athlon X2 4400+.. makes a huge difference when playing BF2