If your mobo can only go to PC2100 then that's as fast as it will go (it's a clock/chipset thing). So putting in faster RAM is pointless, unless it's actually cheaper to put in a pair of pc2700 for whatever pricing reasons (more stock, more production, etc). But going any higher would most likely cause instability and BSODs. For example going to PC3200 (i.e. DDR400) would cause the RAM to run too slow for it's intended design and it will most likely be unstable.
Cheers.