Are you thinking about a compound or stereoscope? The former will give you the option of much higher mag if this is what you are looking for. For looking at samples like blood cells I would recommend getting a compound scope since a stereoscope just won't allow you to resolve individual cells, it will just look like a smear. On the other hand, with a stereoscope you get the option of adjusting both overhead and underhead light which is very useful when you are looking at samples that do not transmit light well when illuminated from below. In other words, what scope you will get will very much depend on what kind of samples you are interested in photographing.
Make sure you get a scope with a trinocular head. Then you can get an adapter to your Nikon
D90. Go to Martin Microscopes
) and get a DSLR adapter to a scope called MM-SLR. This costs 495. This will fit most scopes
but once you know what scope you are getting you can ask them to make sure. The adapter is easy to use, screws on like a lens on your camera body and then attaches to the scope.
I would also suggest getting a remote trigger for your camera since any vibration from you as you hit the shutter button will translate into blurry images.