1. NO! the Oregons batteries are not charged when you plug in the car charger. This is normal with 'most' devices that use 3rd party rechargeable batteries. Only devices that use specific batteries recharge like this. There maybe exceptions of course.
2. Garmin specify 4Gb to be the ‘Tested’ maximum size. However, I am using an 8Gb Class 4 with no problem and someone on the Oregon Wiki is using a 16Gb. Remember to use a minimum of Class 4 speed.. This is a Garmin recommendation.
My blog may interest you if you are thinking about getting a 550T:
