Here is a couple of the better low-cost nav systems:
http://www.carnav.com/index_products.html
If you start getting into the more expensive systems you may want to consider the stock NAV system. I read where many guys recommend buying aftermarket systems; being better of course than the stock system. This of course, is purely someone justifying his or her mistake cause this system is flat good.
The NAV system itself is quite good (more on that latter), but combined with the A/C, Sound, Trip, and rear view camera the system not only is excellent, it is really nicely integrated into the automobile. The integration of the technology system into the the functionality of the rest of the systems is a difficult task, but Nissan excuted this as well as ANY other aspect of this vehicle. The rear-view camera is spectacular; backing down the driveway making sure to avoid cars and children, hooking-up a trailer, backing an empty trailer down the boat-ramp, and the markers indicating distance from the vehicle is just an awesome feature of this truck.
The NAV system is an excellent system on it's own. The Bird's Eye view is a great way to follow the roads as they come into view, with or without automatic routing. The routing computer works extremely well too. Gives great detail, and has great tools like "Quick Stop" for ATM's or restuarants or gas stations.
It's not perfect of course. Annoying disclaimer everytime it turns on, no touch screen, and doesn't let you enter routing information while on the fly. But it's a top rate system none-the-less that integrates really well with the rest of the vehicle.