I can actually answer some of those questions, Arch2ngel. Amazon's requirements for cover images are:
Horizontal: 500 pixels minimum
Vertical: 1280 pixels minimum
Aspect Ratio: 9:11 displays best
Resolution: 72 dpi
Color: RGB
Format: jpeg or tiff
Internal image requirements are more vague:
Format: jpeg, gif, png, bmp
Size: 127 KB max
But yeah, 500 x 700 sounds good.
Barnes & Noble's cover image requirements are much less specific:
Sides: between 750 and 2000 pixels
Format: jpeg
Size: 5KB to 2MB
As for color images and Kindles… I'll let Steve answer that.