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
through the YPbPr,RGB and the VGA2 HD input

and SD video
through the YCcCr

it also had a DVI-D input

1) what is SD video?
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

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

Thanks for your time..
Component is effectively the same as HDMI, but carries video only and no HDCP issues. Don't sweat this, just use the component hookup and be happy...

FWIW, you can easily get an HDMI to DVI-D cable or an HDMI to DVI-D adapter.
SD is standard definition - old-fashioned TV like we grew up with.