Pentium II Or Celeron?
This is a question, which is very easy to answer, the only reason to buy a Pentium II for a dual processor system is its higher bus clock. But in my eyes the difference is too small to spend almost the double for approx. 5-10% more performance, which you will hardly notice. For the price of two Pentium II 400 CPUs you can get two Celeron 433 PPGA CPUs with converter boards and you will still have money left for at least some extra memory. Even the cacheable area of the Celerons (300A or faster) is 4 GB as well.
High end dual processing was never as cheap as now with the Celeron. You can take most CPU converter boards and modify them or go for the new Asus or MSI types in the first place. If you want to run 100 MHz right now, you should stick with the Asus S370, which enables you to increase the CPU's core-voltage, thus making it perfect for overclocking. I've not experienced any problem so far at 100 MHz FSB, but it's never wrong to be careful! Of course you should take a closer look at the motherboard's BIOS; make sure that is not older than 3-4 months and supports the Celeron CPU family, including the right micro code!!!
If an upgrade should become necessary, you can still get either a faster Celeron, a Pentium II or Pentium III CPU. The only thing you have to consider is getting a 100 MHz capable motherboard (BX chipset) and if possible PC-100 memory at the same time.