if you are sticking to 100 MHz FSB processor you are not likely to improve the performance a lot, about 15~20% maybe. that is at 70% MHz improvement.
your PC133 memory will work on 100 MHz systems as a PC100 memory. its backwards compatible, but if your motherboard supports 133 MHz FSB you might be able to run it at 133 MHz and processor at 100 MHz. many chipsets allow that.
<font color=red>No system is fool-proof. Fools are Ingenious!</font color=red>