A jpeg image does not lose any quality if you view it, or open it in a postprocessing software program and don't modify or resave it.
The difference between NEF (Nikon's version of RAW) and JPG, is that your camera uses it's setting that you specify to take the raw image, add the settings you choose and output the image as a JPG. The advantage of JPG is that any computer operating system can view the images and the images you save will be smaller than NEF. The advantage of NEF is that you have total control over the image through software postprocessing. I have used Capture NX2 and am just now learning Adobe Lightroom 3.3. Both do a great job with NEF. Capture NX2 is a Nikon product and can do some very magical editing things with little effort. Lightroom has a cataloging feature that is powerful.
I use the D90 and shoot NEF+JPG Fine. That way I can download the JPG's into a computer and just view the images without processing. This is especially good if you are using someone else's computer to view the images. When I get seriious, I use Lightroom and only import the NEF files.
Bottom line, NEF gives you total control of your images and usually better quality output as you convert NEF to JPG within your postprocessing software.