Yep, I can vouch on the 4.0 vs 3.0.. incredible amount of power.. you gotta supercharge a 3.0 to hit with a 4.0 can do
As for the Extended Cab, yeah.. I would have to agree with looking fo rthat, gives you more legroom if you need to scoot yoru chair back, and an excellent spot to store any gear you dont want weathered. I think I'm personally done with ever owning a regular cab, its just too small.
Good luck with it!! And definatly look for a 4x4 4.0