Changing AMDs FSB speed

Can an AMD XP running at 333, and 400MHz FSB run at 266MHz?
My interest is upgrading a dual mb only capable of 266MHZ
with dual Barton CPUs FSB running at 266MHZ.

Lowering the bus speed will also lower the clock speed. You'd need CPU's with adjustable multipliers. I'm not sure if Athlon MP's have adjustable multipliers or not. You can't do dual without an Athlon MP, unluss you change the Athlon XP to MP by closing bridges on the top.

You could get Mobile Athlons, since those have adjustable multipliers, and make those into MP by joining bridges.

But! You can get the XP-M cheaply, it's already unlocked, and these are actually a better grade of XP, the best of the batch. AMD selects them that way to assure they run cooler and are stable with lower voltages. Overclockers like them because those same attributes allow them to overclock further.

Two XP-M's, converted support MP, would be prefered over two standard XP's, unlocked and converted to MP.

This brings me to an important question that I havn't been able to research yet about how a mb bios recognizes a locked designated (AMD/Intel) CPU speed. The Tyan Thunder 7 series supports AMD CPU's up to the 2800+ model with a bios update. Any thoughts?
I know you might be thinking why bother with XP's when the 64bit CPUs are out, but these are for testing server cluster configurations and the AMD XP's offer the low cost solution with all the power I need for the environment they are in.