The only "legit" ECU upgrade (this is a re-flash rather than a chip, Nissan ECU's don't do chips) that I know of for the 03 Maxima is done by Technosquare. I think they do a pretty good job, and the work is turned around very quickly.

Of course its also possible to purchase piggyback fuel tuning equipment, get it installed, and find a fuel tuning/dyno shop that knows Nissan engines, but that can add up to a whole lot of money (and, typically, a fair amount of frustration before all is said and done) - more than most people anticipate when they set out on this path.

Brian C Catts
Cattman Performance