1) As far as I know, all TN panels are 6-bit. They have faster response times and are cheaper, but they suffer from off-axis viewing. Some HDTVs use higher resolution panels, like the Toshiba Regza with 10-bit and some Samsungs with 8-bit:
http://www.dslreports.com/forum/r21524580-LCD-TV-Panel-Type.
2) The question makes no sense as it stands - do you mean, if two sets are priced the same, but one has VGA input, should you choose the one with VGA input? The best connection is digital - that means HDMI or DVI, and next best is VGA, component, s-video, and then composite.
3) More bits = more colors.