I strongly recommend NOT buying a "generic" car cover that is made to fit many different kinds of cars, such as you will find on store shelves at Costco and elsewhere. Get a cover that is custom made for the NSX. The problem with a generic car cover is that it doesn't "fit like a glove" at all. It will have areas where the cover is loose, and where it doesn't follow the contours of the car body, particularly around features like the side mirrors and the rear spoiler, etc. The wind will blow these parts of the cover around the car, and they will move against the paint. With time there's a significant risk that small dirt particles will blow in and the continued action of the cover will scratch the paint finish.
Car covers are made in numerous different fabrics, and some of them are great for moisture protection (letting moisture underneath evaporate, while preventing rain from getting through to the car). Check out the websites of the car cover vendors for the advantages and disadvantages of each of the available fabrics, and choose the one that's right for your needs.
[This message has been edited by nsxtasy (edited 20 December 2002).]