I had to do a couple of best man speeches and personally, I find it better to speak impromptu...and with a little alcohol in my system. Dutch courage, man, Dutch courage.
I usually feel more comfortable just talking without much planned, but having a vague idea of what to say. You obviously want to say something heartfelt about the couple and so I'd build around that. Blend a little humor to keep things light while staying classy. Most importantly, remember it's their show...which is why you have a few minutes to be the showstopper. Enjoy it.
The 'best' Best Man speech I've ever seen was when this guy, with a full glass of wine in hand, said "I'm very nervous right now" and then, had gulped it down. Classic.