Perfect lens and the lighting set-up sounds ideal. The light off the ceiling, soft box and plexi all add to the required diffused light that is best for shooting watches.
There is no need to shoot on 800 ISO especially if you are using a tripod. I always use 200 ISO and wish it went lower like the old days (Ektachrome 64). I would recommend one more piece of kit - a cable release. Nikon MC-30 will fit the D300. Use this with the mirror lockup function (Mup) accessed via the ring on the top of the camera. Its a 2 stage process - first firing the mirror goes up and second firing activates the shutter. This reduces camera shake to an absolute minimum. A good tripod also helps.

I don’t bother with white balance as I can adjust colour temperature in Photoshop but it may be worth experimenting with the various setting and your lighting set-up, which I assume will remain constant. I also suggest bracketing exposure. As it is digital multiple images costs nothing. Set the camera to manual and select the desired aperture. Use the light meter to determine the correct shutter speed and take shots at the "correct" speed, slower and faster speeds. Select the picture with the best detail especially in the highlight areas. Use levels to improve the contrast. Finally, sharpen to the max i.e. just before it looks overdone on screen. If you print images, you can sharpen even more.

I am not that familiar with Elements but I believe that it has many of the functions of the CS package. Like any software, when you get to that comfortable familiarity it become indispensable. Just be sure to use levels and you will see immediately why.
Regards
Paul