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