You're probably right, I think 33MHz was the upper limit even: Info here (PDF document).
-edit-
Holy crap, it's tolerant to over 1M rad (10,000 Gy)...
What is a more beautiful thought (for me at least), is that once the Earth and human race have been destroyed by our dying sun... one of the few things that will survive is the odd little RCA1802 CPU that has faithfully powered the Voyager spacecraft for over 30years...
It could well be that the two Voyager's will be the only evidence of our ever having existed... It's like a gothic romance in it's beauty