TV inputs

Hello my 42" plasma built in the stone age of plasmas (2002) doesn't have a HDMI input but my hd cable box does
and this tv supports HD &SD
1920x1080/60i
1280x720/60p
720x480/60p
720x576/50p
through the YPbPr,RGB and the VGA2 HD input

and SD video
720x480/60i
720x576/50i
through the YCcCr

it also had a DVI-D input

1) what is SD video?
and
2) would a HDMI adapter to the dvi-d or the VGA2 hd input be an upgrade from the component video input and/or the s video input?
or is it just a waste of time?
The tv only has one component input and I do have the matching control box for the tv that offers
1)GBR/YPbPr and a 2nd with only YPbPr
but like to not use that box as it's having issues with the remote picking what the remote controls the box or the tv it should when used bypass the tv remote eye but it's not at the moment.. and adding another piece to the signal most likely won't help now, it did when formats where limited and the pixal plus cleaned up low res. formats but today it be a step backwards I'd think..
like to have a 2nd good video input
as it stands now there is the 1 component video in use
1 composite video in use (a/v menu )
and 1 s-video used from cable box for use without the surround receiver on.
no idea on the video quality stepping stones as far as
where the DIV-D AND VGA2 hd fit into the list, I know that these go from lowest to best
old coaxal cable
composite
s-video
component
HDMI

where do the Dvi-d and VGA and VGA HD fit in that list?

Thanks for your time..