Penn is such a piece of shit lol, the guy talks so much shit and is so full of himself it's ridiculous. I always root for him though, he's living proof that technique almost always overcomes size and strength. IMO if he is in as good shape as he says he's in, he will convincingly beat GSP. If not, GSP might take a decision. As much as I love GSP, and as awesome as GSP v. Anderson Silva sounds, watching Penn beat him, move back down to 155 to beat Florian and move back up to 170 to beat Alves sounds so much better.
Also I'm going to go ahead and say Machida def. Silva via KO.
Thiago Silva's style is tailor made for a Machida knock out. He likes to push the action and wildly overrates his own stand up. He's never fought a decently skilled opponent, and he's going to be face down on the mat if he tries to bullrush Machida with his sloppy stand up.