Tips for photographing indoor volleyball game?

I was assigned to take photos of a volleyball game at my college. It was held indoors in the gym with florescent lights. I had some trouble with this because everything moves so fast and the lighting situation is less than ideal. I didn't use flash because it was considered distracting. I tried many shooting modes, in shutter priority and in program, but I just couldn't seem to get it right. The photos turned out dark and blurry. I know I need a faster shutter speed, but it wasn't possible with the low light. All I could think of was to bump up the ISO, but that made my images very grainy. I got a few decent shots that I'm able to use, but I was disappointed in most of the other ones.

I have a Nikon D80 camera. I'm still learning about photography, so any advice for handling this tricky situation would be greatly appreciated.

Thanks!

Added (1). Lens: AF-S Nikkor 18-135mm 1:3.5-5.6G ED
It's all I have for the time being…

If you're zooming in at 5.6 there isn't much you can do short of paying $7000 for a 300 2.8.

You are indeed pushing at the limits. Try to borrow a faster lens, i.e. One with f2.8 or f1.8 aperture. I have done pics in a gym with averagely bright lighting, exposures at f2.8 were around 1/50 second at 1600 ISO, and, yes, they were grainy. Try to photograph at the peak of action, with that tiny moment of stillness as the direction of movement changes. At least with digital it costs you nothing to shoot lots.

You simply don't have the gear to do well at this task. If budget is an issue, you might consider getting a 50mm f/1.8 lens. It will not get you very close to the action, but it is fast and sharp and will allow you to use a faster shutter speed. It will also autofocus on the D80. The D80 does not have good high ISO beyond 400, (or maybe 800 if you do not under-expose.) In digital terms, the grain is called noise.

Bump the ISO as high as you can, you will get grain/noise live with it. Borrow a faster lens, an 85/1.8 would be ideal but if you must you can use a 50/1.8 and crop the images. Depending on your intended purpose you should be OK. And BTW, those great Sports Illustrated shots for covers are shot with flash, LOTS of flash, MOUNTAINS of flash equipment in huge indoor arenas (but not on camera).