Uses Pc2100 Memory 266 but want to upg...

No , you may not. According to your M B manual you may use up to 2GB of 200MHz, or 233MHz or 333MHz. Not the 400MHz.  what wil usually happen if you attempt to use the other is that the memoryt wil be too fast and the computer will hang or freeze.

http://support.asus.com/download/download.aspx?SLanguage=en-us

The fastest speed that the motherboard you have takes is PC2700.  BUT... if you stick that ram in with the PC2100, the speed will automatically adjust which makes it backwars compatible, so if you put that in WITH another 2100 ram, it will downgrade itself to the matching speed of 2100.  But you have to have this other ram in for it to work... Be aware, you can still upgrade to PC2700 however...  Hope this helps!