I have an Athlon XP 2000 + (MS-6561) 256Mb of RAM and a I tried to add 1Gb RAM PC2100 (2 slot of 512) but when I turned on the SO, the pc doesn't see it.
Why ?
First off remove the RAM you just added. Restart your PC, then head over to Crucial site and run the system scanner. That will tell you what RAM is compatable with your PC