I had to take the pc in and get the hd formated (hate XP) and reinstall XP the repairman suggested I upgrade to 256 (original 128) he says XP should have 256 he says I need DDR mem I went to pricewatch and LOOKED at mem but I noticed some of the listing had 2100 and such in front of the DDR 256 listing what does the 2100 stand for and which do I need? :-/I'm not sure about the 2100,  but I just purchased more memory from  ArchMemory.com     you can enter your computer info an they show you exactly which memory card you need. 128 and running XP? It's possible but a memory upgrade is good advice....

The 2100/2700/3200 etc is simply speed based, higher number is BETTER......

another thing..... although he said 256 is what you need to have.... it is always better to get more.... I recommend 512....

You say you are wanting to upgrade to 256? all you need is another 128 stick

I've ran 128 on XPpro and it does work, even 64 works....
i reccomend at least 256MB of RAM to RUN Xp Pro...more is always better but at 256....make EVERYTHING run smoother than at 128MBThe kind of RAM you want depends on what your MOTHERBOARD supports. Tell me that and I can find you some really easily. ^^