ive heard tony say this quite a few times... mafongoo mafungoo or something like that.. what does it mean?

I could be wrong here, but I've seen it spelled "ba fangool", and been told it means "go f*** yourself". Someone else told me it means simply "f*** you"

It also comes up in GF I, during the big dish throwing fight between Carlo and Connie. I may have this backwards, but I think Carlo says it to Connie, and then she says "Oh, ba fangool you". It might have been the other way around; I'm not sure.

I'm not Italian, so correct me if I'm wrong, but it's pronounced "ba fangool" but I think it's spelled "vaffanculo," which means "go to your asshole," or "go f*ck yourself"

However it's spelled or pronounced, it means just what everyone said: "go f*** yourself!"
Derivation is from "a tan cul," or "up your ass." If my paisani here will forgive me for repeating a story I posted earlier:
I used to work for a very large corporation that had extensive grounds at their HQ location. They built a bocce court for employees to use on lunch hour. A league formed, and the finalists were a team of WASP middle managers vs a team of Italian-born groundskeepers. The WASPS won. I was the editor of the company newspaper and we had a column that included news of clubs that the company sponsored, including the bocce league. The captain of the WASP bocce team sent me a blurb on their victory. He called his team the "Fun Ghouls." I had to call this guy and tell him I couldn't run his blurb because his team's name was, in reality, an Italian obscenity that he and his teammates were unaware of. Was his face red!

For a proper spelling of that particular italian "colorful metaphor" see my tag line below