Even if they change the body, and the drivetrain, the 03 & 04 M45's are still a smokin' deal right now. In 2005, if the imrpovement is vast, the used 03 & 04 models will just be that much of a better deal. If I want to get to 60 MPH much faster, I would buy a G35 coupe.
I think the improvements will be cosmetic, and mostly the interior room increased, and the engine/drivetrain will probably be de-tuned, if anything. Nobody really raved about the performance. There we a lot of negative comments about back and front seat room, so if Infiniti is listening to the car mags, that will change.
I think any larger of interior will make a wider car, or longer, and that is not what I want. I like the proportions of the M45.