The new model should be at least as capable. There's no word yet as to any horsepower increase, but Nissan usually launches new models with a slight nudge in this area. One thing we can probably count on is a version of the new 5-speed automatic with sequentially shifting manual-mode currently found in the G35 sedan, replacing the old 4-speed transmission. The 6-speed manual, available in the SE, will most definitely carry forward into 2004.
One of the best things about getting down the road in the current Maxima is its minimal torque steer, the bane of most front wheel drive cars. This stability, as well as the steering system's ability to feel the road and respond instantly to direct input, shouldn't change with the new model. The outgoing car brakes with equal control too, due to ABS assisted discs on all four wheels helped along by electronic brake force distribution (EBD). Most likely traction control (TCS) will still be standard on automatic equipped 2004 models while the 6-speed manual will continue with a helical limited slip differential.
It would be hard to believe that its handling could improve but it probably will. The next generation car will share its FF-L platform with the new Altima, and it's no slouch around the corners. Still Nissan wouldn't be satisfied if the new Maxima didn't improve on its lower priced sibling's good manners.
While the current car's interior still looks good, an update would be refreshing. The current car is more upscale than other Nissan sedans, sharing much of its interior trim and switchgear with the Infiniti I35. This trickle down effect will most likely be enhanced with the new car as current Infiniti models have been improving their interior plastics quality, fit and finish and overall functionality with each new model.