I put in 1GB of DDR 333 RAM into my Emachine and it worked with no problems.
Because I ran out of slots I had to remove the original 256MB of RAM. It was DDR 400. While I feel bad that I'm slowing my system down I'm glad I can recycle the RAM that I bought (second time around) to go into my outpost.com machine.
Because this 256MB RAM is DDR 400 I can add to the 512MB of RAM currently in Mattern PC when I get back to Indiana. That should be enough to keep it going for a good while.