A couple of Kiwis tried to visit every country in the world in 6 months. They missed on 2 IIRC. They did a book about it,
An Absolutely Outrageous Adventure by John Bougen. I had a copy but gave it to another FTer some time ago.
As for slower paced (but still relatively fast) there are quite a lot of FTers with the goal of visiting every country. For example see (
here,
here 
,
here).
A few sites you may find helpful include passportstamp, mosttraveledperson, world66, etc. I have a series on my travel blog of
"how to get to" various countries and places (eventually will post about every country - a long way to go at the moment). You might find that useful.