I'm assuming you're referring to the the DRAM settings in the BIOS? That sets whether or not you have PC100 or PC133 memory. You've said that you have PC100. PC100 is the default setting, so leave it as is. Your system will most likely crash completely or show a lot of instability if you change it to 133.
As a side note, if you have two sticks of RAM they should be in the DIMM 1 and DIMM 2 slots. This will provide for the best performance.