Your post really caught my attention because I've been considering exactly the same thing. Moving to a rangefinder has some really attractive benefits. Like the smaller, less intrusive size, quieter noise, and sooooo much more convenient for travel and candid work. I also use a Nikon 35mm, and I regularly compare results with a friend who uses an Olympus OM2 with the same slide film. He consistently gets better colour rendition and contrastier results than my prime lenses in exactly the same situation. I can only assume a rangefinder without mirror movement and significantly faster lenses will consistently get better results. And the Leicas are such beautiful cameras - definately an attraction.
Lees comment is something to consider .... "It's not as versatile as an SLR, however, and I would suggest you keep your Nikon to be able to use long lenses, do close-ups, etc". Fast autofocus and long focal lengths are the things I'm personally scared of giving up. I believe the Leicas are limited to 135 or 150mm max (?) So nature/wildlife photography could be more challenging - if this is a consideration. There are a few photographs I'm glad to have (and I'm convinced I would not have, if it were not for the Nikons' excellent spot metering and fast AF).
Would be very interested in hearing what you decide. Good luck.