an amp must be coupled with an appropriate antenna for the job. An amp could be likened to an electric megaphone, sure, you can shout further, but the person you're talking to must be able to should equally loud for you to hear it. A directional antenna can be likened to one of the old-school pep rally megaphones. You can yell through it, and be heard louder, but you can also put that megaphone to your ear and hear far away things better. However, for automotive use, you want to be looking for signals all around you. A directional antenna would not be appropriate.
An amplifier could assist in picking up weak signals, however, it may not. An amp would only be appropriate if you are having excessive cable loss to the antenna (which if done properly with correct cable and connectors, should never be the case). That being said, it would probably not be worth your while to spend the 250+ for an amp. I own a 1 watt amp and a .5 watt amp. I use both amps coupled with various different antennas, both omni and directional, depending on what I'm trying to achieve. I will most probably not being using an amp with my carputer wifi setup because my antenna will have enough gain to hit most APs. However, if I complete my stompbox-like project, I may use an amp with it.
Another important thing to consider is the whole legality aspect.... The FCC code for wifi transmissions is that you are only allowed 4 watts of effective radiated power. I don't remember the formula offhand to determine what your radiated power is, but IIRC, for 500mw of power, you can only have something like a 8db antenna. It's not that much. I just found a page that has a couple handy charts to go by. http://www.wifihowto.org/?mo=HowTo;Item=9 Only the first chart would apply to automotive use, UNLESS you use a yagi, dish, grid, or similar HIGHLY directional antenna.
Wow, what a mouthful. Hope this helps you in your quest for max range of wifi.