As noted above, don't buy more than you need. An XP box from 2004 may be limited by the motherboard to 1024MB (1GB) of RAM.
XP itself is limited to 2GB I believe. If your motherboard will take 2GB, just buy one 1GB chip and go up to 1.5GB. You'll notice the difference.
Mainly take care to not static electricity the crap out of the crap or your computer.