No, you dont have to stay with PC2100. Yes, you can use PC2700, or PC3200. If you put higher speed RAM into a system that either doesnt support higher speed RAM, or the RAM you already have is slower, then all RAM will clock itself to the slowest component.
So, if you add PC3200 or PC2700 RAM to your system, it will underclock to PC2100.
THe reason I asked what kind of motherboard and CPU you have is because if you have a system which can actually support PC3200, then you might want to consider getting rid of your PC2100 and just getting PC3200.
But, PC3200 seems to be more widely available, and I recommend getting it regardless.