PC 3200 is USUALLY backward compatible, but it depends on the motherboard. If all is OK the 3200 will operate at the slower speed though, so is there a specific reason why you're going for PC 3200 rather than PC 2100?
pc3200 will run with the pc2100 with no probs but as MAJ says it will run at 266mhz and not the 400mhz at which the pc3200 runs at. If the 3200 is cheaper it is the best option as later on you could by another stick of pc3200 if you wish and ditch the pc2100 off to ebay.
I see what you mean, sharpamat. It's impossible to say if the PC3200 will be backward compatible in your specific system, although as I say, it usually is. It might be better to phone Crucial and ask, they'll know for sure or give you a guaranteed replacement if it doesn't. Freephone 0800 013 0330