"Does that mean that its memory is full quickly?"
exactly. set up an xp computer with a minimum necessary for autodefense (a firewall and a resident antivirus). boot and have a look at the task manager - 130megs of memory already taken.

when you play games here, you use a browser; how much memory that takes depends on what browser you use, and how many sites you visit. if you go and visit sites that use java, or flash, or pdf pages or... your browser will load each one in turn. and even if you unload the pages, some of the allocated memory remains resident. xp is better in this respect than some of the older microsoft marvels - once you unload the browser, you recover most of the memory that has been allocated.

still, don't bother with xp unless you have 256 megs. and even so you'd wish for more.