Go higher if you can, they will generate errors long before burning out from speed. Raising the voltage is where you kill them and with crucial 2100 they can take a good amount of voltage before you damage them.
I got my pc2100 Crucial to 166 with only +.10 voltage. I hear without a heatsink, safe operating voltages are about 3v. but, if your going to be running at that voltage for very long.. GET AN ACTIVE COOLING SET!