Hey,
I don't know what you have in your cigarette lighter socket right now, but if you aren't using it, you have another solution:
I used to have a nokia 6100 series phone (just a few more features than the 5100). I was able to buy a kit that plugged into the lighter socket. It provided power, and had a built in speaker for hands free. The mic clipped to the sun visor, but had a 10 ft cord or something so that I could run it along the roof liner, down the front passengers' side plastic liner thing (duh... can't remember), along the airbag crease in the dash, right into the unit. It looks a little messy, but saves you from doing a permanent install.
I have one now for my newer cell phone.
As for a holder, I had some difficulty... for a while...
I keep my phone in a leather case w/ a belt-hanger on it.
I wanted a holder that would take the phone IN the case. I found a mount with 3m tape on the one side (to the dash) and a magnetic plate on the other, which pulls on the metal in the case and battery. It's really secure. Surprisingly so...
Anyway -- hope it helps,
Brian