Well, yesterday I had a single generic stick of 128M PC-100.  Today, a 256M stick of PC-133 arrived, also generic.  Both sticks are in the computer and things are running great.
Some people may say that RAM is faster when using only a single slot.  However, I figured that the motherboard has multiple slots so I am using them.  The only reason I can think of to just use the new stick is if you want to clock above 100, and are concerned about the old 100 stick causing problems.
Jeremy