Yeah, DEC Alpha was the first thing to come to my mind. I guess some might argue it wasn't a "PC" - if you don't include what used to be called "workstations". This is largely because PC meant "personal computer", and very few people could afford their own DEC Alpha - they were very pricey ($20k at some point in the 90's, I believe).

It even has 64 bit "word" size!