The major difference between the NX1600 and the NX2000 was the size of the engine. The NX1600 had a 1.6 liter I4, while the NX2000 got the larger 2.0 liter I4.
Although similar to the Sentra SE-R, the NX2000 was upgraded for better performance. The brakes were bigger, the wheels were wider, and the tires were aimed for sportier driving. Many NX2000 parts will fit on a Sentra SE-R with minor modifications, and these parts are very popular aftermarket upgrades.
Not only were these cars lightweight, but with a stiff chassis and a limited slip differential they were built for hard driving around curvy roads. Many consider these cars to be the best-handling front wheel drive vehicles ever. Road & Track even included a NX2000 in a test of some other good handling cars, all of which were high priced sports or exotics from the likes of Porsche.
These cars are very popular in the import community. They are very popular tuners. If you would like to buy a used NX the best place to look is on eBay, where you can find some listings with aftermarket mods already installed. Original cars still go for cheaper than in most other used car listings, and parts are also readily available if you want to mod it out yourself.