No.
Or to be more precise, I don't think that all 800Mhz mobos will run at
lower speeds.
I might be wrong, maybe there are some 800Mhz CPUs that won't run
at lower Mhz, but I do know that I have seen something about such restrictions in the last
6 months.
Certainly, you need to be carefull. I think I'd look for the slowest
P4 that does run at 800Mhz.
Peter
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PaQ