It all depends - maybe - maybe not.
If your motherboard is set to run the RAM at 100MHz (even tho' it's get PC133 RAM in there, it may be set to 100MHz) - then adding the extra stick in may work.
If your motherboard is set to run at 133MHz - then the xtra RAM definately won't work, unless you drop the RAM speed to 100MHz.
Even then, some systems are fussier than others when it comes to mixing RAM speeds, and even makers. There are other parameters besides the basic speed, and they may not be compatible.
So - the ideal is to have two sticks of identical RAM.
The next best is that they won't work together at all (at least you know where you stand)
The worst is that they appear to work but give you strange intermittent errors.
Needless to say, RAM timings are adjusted in the BIOS.
I have long been of the opinion that many starmge and intermittent screw-ups are down to mis-matched RAM.