Does an external hard drive attached to the camera for capture improve the quality of the film? In other words, if I stop using the miniDV tape in my GL2 and instead record to a product like Quickstream will the image fidelity improve? Is it a viable substitute to a tape deck?

No, it is the same information. Nice alternative to tape but not, potentially, as reliable. Certainly much more expensive on a per-byte basis.

That's right, there's no improvement in quality. Also, generally, it would be a good idea to leave the tape recording even if you are recording to an external device. It's a good form of backup, and provides some peace of mind that in the event of the worst happening and the drive failing, you still have a taped backup.

From what I have learned, I am going to get a tape deck.

Why are you going to get a tape deck?

I don't want to kill the heads on my camera and I would appreciate a dedicated machined that is always hooked up and runs faster than a digital camera.

To the best of my knowledge (and I've never used a tape deck)
it will not run faster than a camera will. At least not while
capturing. That's still at 1x speed. Rewinding etc. might be faster,
ofcourse.

The reason I asked was that you wanted to know wheter an
harddisk improved quality. So I was worried you might be thinking
a tape deck might do as well.

Indeed it can be a very good investment to safe your camera's
heads and have a dedicated deck with your NLE and most can
even be used for analog in/out I believe.