Your new Samsung plasma is BEGGING for an XRGB Mini! I have a Samsung as well, a 50" plasma though it's now about 4 years old. Still an impeccable picture. (Very fine and wise choice on going plasma over LCD/LED also!)
So my TV does have a VGA input, and I use the (official) Dreamcast VGA box directly to the TV. It's one of the very few consoles I don't run thru the Mini, so sadly I can't help you very much there - other than to say yes, you will need some kind of VGA -> Component converter.
Gamecube via component is only available via the expensive official cable. I'm actively looking for one, and yea I'm not looking forward to paying for it. There are no 3rd party or aftermarket Gamecube component cables, unfortunately. For now I'm using S-video from the Gamecube.
D-terminal adaptor may not even be necessary since your tv probably has component inputs. However, it's a good way to future-proof your Mini. (Perhaps your next tv may not?). If you buy the D-terminal adaptor from Solaris Japan along with the Framemeister, you'll be ready to run any console via composite, S-video, component and RGB for the rest of your life so long as you have a TV with an HDMI in (which will likely be a standard for a long time).