That computer will take two modules of DDR PC2700 or PC3200.
You can get the two modules today at
www.crucial.com for $27.99 each with free shipping... or at
www.directron.com for $26.99 each plus $6.75 shipping... and other such sites. It is best if you put in PC3200 and take out the old PC2700 memory.
The maximum capacity is 2 modules of 512 each for a total of 1024 mb.
Then when it arrives, don a pair of rubber or plastic gloves, remove the existing memory by pressing back the two white or beige clips, one at each end. of each module.
Then carefully, but firmly, insert the new modules, after verifying that the slot on the bottom edge is properly lined up with the position bar. Force the memory into place until the two little end pieces snap into place.
Be sure you have good working space and a very strong light before you start work.
Good luck.
The 4400 is a good solid computer that can last a long time if you keep the fans clean... so while you are working, use a can of Dust Off, and perhaps a couple of Q-Tips and some denatured or 95% rubbing alcohol to clean everything gently, carefully.
