I've actually spent lots of time (maybe too much) trying to find a car charger.
From what I've been able to gather, its not quite as simple a decision as you might think. The question is the charging current. The higher the current the faster the phone can charge. But you shouldn't exceed what the manufacturer recommends... which of course isn't documented.
The USB standard says that USB ports on a computer MUST be able to provide 500ma of power (1/2 AMP) @ 5V. But this isn't much. Many manufacturers, possibly starting with Apple, upped the amperage with their chargers to 1A (1000ma).
Recently Apples iPad needs more current to charge it. It needs 2.1A (2100ma) to charge. New car chargers are just coming on the market that support it.
So the question is how much current can the Incredible handle to charge it up in a car? The supplied AC power adapter outputs 1A so it can handle at least that. I've found references online to an HTC OEM car charger which outputs 2A. So it seems that it should be able to handle 2A. This charger should be able to charge it up to 4x faster than a regular 500ma car charger. But this HTC charger seems hard to find.
I was actually planning on going to the Apple store and picking up an aftermarket Griffin or Incase iPad car charger. Is has a USB port which lets you plug in the cable from the Incredible to charge it. Its relatively expensive, $25, but if I'm charging in the car, I going somewhere and often need it charged up as fast as I can. This charger should work for iPod, iPhone, iPad, my G1 and Incredible.
Edit: I should mention that I am not confident that its ok to use a 2A charger with the Incredible since I can't find documentation anywhere from HTC. I do plan to try it after I get my phone on Thursday.