> would it be possible to take the motherboard (+CPU, +RAM) from an Ultra10
> and plug it into the case of an Ultra5 ?

Yes, that is possible.  It's the same motherboard, and even OBP
does not know if it is an Ultra 5 or Ultra 10... :-)

> - because CPU is 100MHz faster and IDE bus is 66MHz rather than 33MHz (according to eBay and google's dmesg).

That is strange.  It's possible that there were different boards
with different speeds, but I rather doubt it.

Maybe the Service Manual tells you more:

http://fr.sunsolve.sun.com/data/805/805-0423/pdf/805-0423-11.pdf


HTH -- Volker
-- 
------------------------------------------------------------------------
Volker A. Brandt                  Consulting and Support for Sun Solaris
Brandt & Brandt Computer GmbH              WWW: http://www.bb-c.de/~vab/
Meckenheim, Germany                                   Email: vab@bb-c.de