If I get a decent mic, can it eliminate this hiss? ... Can either of you audio techies recommend something?
Yes ... change the laws of physics!
If you were to buy an NTG-3 (and learn how to use it well) you would certainly reduce the hiss you are experiencing currently. You would still have audible hiss though and, the mic improvement would now highlight the weakness (hiss) from the pre-amps in the DR100. So then you would maybe move up to a SD mixer + recorder, which would reduce the hiss further still but highlight the fact that while the NTG-3 is in a completely different class to what you have now, is still relatively a bit of a weak link. So then you will need to spend say $2k on a top class mic to go with the top class mixer and recorder.
Even with hundreds of thousands to build an extremely high quality recording environment, buy the finest quality mics and pre-amps and position the mic perfectly you still can't eliminate hiss, the very best you can achieve is reduce the hiss to inaudible levels. Of course, on location (or even on a sound stage) you never have an extremely high quality recording environment and virtually never can you get the perfect mic positioning. So even with the finest equipment money can buy and a top class professional production sound mixer it's still impossible to eliminate hiss and usually impossible to reduce it to inaudible levels.
You can see from all this, it's a bit of a dog chasing it's tail situation. Audio quality is dependent on the entire audio chain of equipment and is defined by the weakest quality piece of equipment in that chain. Furthermore, the higher the quality of the equipment chain, the more obvious are the weaknesses (lack of skill/experience) in the person operating it!
While the Azden brand has a particularly bad reputation (Alcove covered it well!) and the NTG-3 could provide a substantial improvement, ultimately you are fighting a battle you can't win. Unless you are planning on specialising in being a professional production sound mixer, maybe spending your money on hiring a production sound mixer would be wiser? It would free you up to concentrate more on the other areas of your filming, rather than having to spend more and more time getting the best out of your audio equipment upgrades.
G
Scotty: "I cannae change the laws of physics captain"