Pixels?

[short shifter]

On my canon A85 i have an option to change the pixel count, which in return allows me to take more or less pictures. So my question is, if im taking pictures knowing i want to have them printed on standard 4X6 paper, what pixel count should i use?
2272 X 1704 or 1600 X 1200 or 1024 X 768 or
640 X 480 . And if i want to print on 6X8 paper? Also i have a postcard option which is also 1600 X 1200 whats the difference between that and the normal 1600 X 1200? I looked in my users manual before asking this question here and didn't get much of an answer.

Thanks for all your help. This is my first digital camera, and trying to learn everything about it. It was highly recomended for its ease of use, so hopefully that holds true.

Thanks for your help,
Jay