CII 950 performs , say, similar to PIII800(Cuppermine). PIII550 (Cuppermine) would probably be around Celeron II running at 700Mhz on 100Mhz bus (although there is no such thing). Katmai PIII is a bit slower than Cuppermine, so you would see a good bump on performance if you are lucky enough to get a CII633 running 950Mhz.
In case your CII 633 turns out to be a dog and doesn't do 100Mhz, and 83Mhz does not work because of your PCI card, harddrive, or something, you will be stuck with 75Mhz or 712.5Mhz which probably only as fast as your PIII500@560 or might even be slower. So look carefully.
Of course, it would not be as much of a problem if your motherboard have more FSB setting between 66Mhz and 100Mhz.