A couple of tips on the BX6R2 (I have one).
If you haven't, flash the QR bios. It allows automatic set-up of up to a 700 MHZ PIII or a 633MHZ CeleronII. Higher speed CPU need to be set through the "User Defined" settings and are not "officially" supported - though they do work.
If you'e installing a PIII which defaults at higher than 700MHZ it will not be recognized correctly and you will need to use the softbios settings to configure multiplier, bus speed, agp ratio, and so on. Once you've configured it, it will report the correct speed at post.
I do know that the board will support a PIII-800E (Mine's runing at 936 MHZ) and have traded notes with some folks running 850E's as well. There are numerous reports of folks having success up to and beyond 1GHZ with a good overclocking CPU, good memory and FSB's up to and beyond 140.
Happy Overclocking!