The 5 volt USB ports on computers provide 500milliamps - that seems to be the standard.
For some time I have been using a 700ma wall charger from an old LG phone that we don't have anymore.
The family now has the latest iPAD and it comes with a 2100 ma (also referred to as 10 watts) wall charger. The iPAD will not even go into charging mode when plugged into a computer - and thus the docking station is rather useless.
A power supply, whether it is a plug in for home use, or one for the car, can provide 2100millliamps, but it is up to the device(phone) to actually suck it in. If a device is designed to only take 500 milliamps, then there will be no advantage of using a 2100 milliamp power supply. On the other hand if the device will take the additional milliamps and charge faster, then you run the risk the battery will overheat. Heat destroys Lithium Ion batteries. In addition, Apple did something proprietary in its charging systems - and not all chargers will charge an iPAD. It's not just enough milliamps, there's something else - something wicked apple designed to try to keep people over-paying the genuine apple chargers.
There is a free app called "Battery Widget" which among other things will display the battery temperature. Keep in mind that the cheap non-OEM brand batteries often do not have a temperature sensor inside them.
Amazon has this for $10 which is supposed to be Apple iPad compatible:
PowerGen Dual USB 2A / 10w (fast) Heavy Duty Ouput Car Charger for Apple iPad 3 "The New iPad", iPad 2, iPhone 4 4s 3Gs 3G, iPod Touch (USB Cable not included) - Black
I'd suggest trying a 10 watt (2100 milliamp) charger, but watch your battery temperature.
Howard