A day or two before I started this latest research binge, I had a chat with a friend of mine from college. I clearly remember him saying, "Research? You used to research the bejesus out of things." This is true. I have a hard-on for research and one would think that I'd found the brain version of Viagra.
Research is important for the writer. You need that knowledge to give credibility to your story, or rather you story world. There's a scene from the Billy Crystal movie Throw Momma from the Train where a writing student read from her war story set on a submarine. I'll never forget what she said: "'Dive, dive!' yelled the captain through the thing. So the captain pressed a button, or something, and it dove." Even Billy Crystal's character said you should know some submarine terminology if that's the setting. Does it have anything to do with the story? Probably not. But if Gene Hackman or Denzel Washington didn't know what an emergency action message was, Crimson Tide would have lost some of its luster.
Now, that being said, I do think there is a point where you can ease your foot off of the pedal. For example, the invasion story I'm writing is set during the Civil War, and God knows how tempted I was to research to the gills on that, but I didn't. Railroads, for example, were largely left to my imagination because I don't have a lot of scenes set on trains, and those that are don't go into heavy detail on locomotives. Trains were vitally important during the conflict, but I doubt many of the guys fighting for either the North or the South knew how they worked. All they needed to know - and for my purposes, all I need to know - is that trains were important.
Also, don't be afraid to take some liberties with your research. For example, looking into the different armies of the Civil War (the Army of the Potomac, the Army of the Shenandoah, etc.), I found out that a lot of these armies got disbanded and absorbed into other, larger units that themselves were constantly changing. Finally, I said to hell with it. If Robert E. Lee had a certain number of corps and divisions at Gettysburg, then that's what he's got throughout my story. And if Civil War buffs want to cry foul, let them.
Research is an invaluable skill for a writer, but you have to know when to prioritize what you need and what you can fool around with. It takes practice to figure out this distinction. In the meantime, while you do work your way towards that mastery, always remember to take deep breaths and keep your head on straight. Otherwise, the reports of your brain-death will not be greatly exaggerated.