Minecraft has "run out of memory" help pelase?

When i get into minecraft i play for about 30 seconds when it saves and says that minecraft has run out of memory, it says it could be a bug in the game or the java virtual machine not being allocated enough memory, how do i allocate it more memory? i am running windows xp, 32 bit, i have plenty of disk space free and i think i have 2 gb of ram.