This a more complex question than it seems.
For the best performance in any scenario, get brand name PC133 cas2 RAM. Cheap as dirt right now. Maybe some of the Virtual Channel(VC) memory being discussed in the hot deals forum.
Much of what was sold as pc66 is actually pc100. The conversion to 100fsb was quite rapid, and true pc66 disappeared in a flash. Vendors sold a lot pc100 as pc66 knowing it was fully compatible and that customers were easily confused. Your stuff might work, hard to say, unless it's older than the pc100 standard.
Via chipsets allow the memory to be run asynchronously with the fsb, at either 100 or 133. Dunno about 66 though.
Is that better, or worse?