The datasheets says it'll run UP TO 40Mhz internally, by using the 4xPLL, meaning the most you can put in externally is 10Mhz.
I've had ONE run at 25Mhz on an external clock (by mistake before I read the datasheet closely), but that's not the norm and most likely won't work for others, not to mention it ran a bit flakey, locked up often, that sort of thing.
EDIT: Ya, what Mr.E said...I really should read first...
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