2+1 gb in xp.problem?

Most newer motherboards have 4 memory slots. The memory slots are different colors if the MB supports dual channel. Each channel needs to match in size to retain this functionality but I don't think you would lose much performance if they didn't. So if you currently have 2GB and each stick is in a different color then you aren't utilizing dual channel anyway. If you want to add a third GB and retain dual channel capability you need to put in two 512MB modules in the second channel (color). I hope this helps and maybe someone can correct me if I am wrong.