But from what I understand, these cables that also have a SVIDEO plug are meant to address precisely that problem. You plug the DC end into the DC, the VGA end into your TV's VGA port and the SVIDEO into the TV's SVIDEO port.
Well, not really. The DC is from 1998, and in 1998 there were virtually zero TVs with VGA ports on them. Usually only projectors.
That's the way I'm understanding it to work, anyway.
The DC outputs whatever the VGA box was set to (either composite/s-video or VGA) when it was switched on. The DC knows what cable it has hooked to it. Remember the "Its thinking" ads? Its true. Its smart. Booting with a SCART cable in RGB is also a discrete mode. The BIOS screen will run in any mode, but beyond that it depends on the game. It can only run in one mode at a time though.
There was a Sega VGA box, not sold in the US. It was 5800 yen or something stupid like that, which is way too much since the VGA natively puts out VGA. Most of the circuitry is just for switching and such. There are a load of cheap Chinese boxes though. Some have headphone jacks, some dual headphone jacks. Some are really picky and won't run with certain displays. This is why I have more than one of them. The connectors are often junky too, which when combined with the DC's already dodgy multi-out port, game weirdness, and picky monitors makes diagnosing a no signal issue a pain the ass.
Once you have it running though...awesome. Completely.
Just so you are prepared, there are a lot of stupid things about the DC and its various video modes. Many many games do not support VGA, and for no good reason I can think of. Some games can actually be tricked into running in VGA by using boot discs like the Super Game Converter. Sometimes there will be minor graphical glitches when you do this, sometimes none. The US version of Street Fighter Alpha 3 will not run in RGB, but the EU version will. The first 2 games in the 4 game Sakura Wars Complete Box won't run in VGA, but the other 2 will.
The 16:9 modes aren't exactly like what you might expect. In VGA mode the DC always puts out 640x480, so when you run a game wide, like F355 for example, you will lose those lines of resolution needed to make the image wide. It just changes the shape of objects on screen so that when you stretch the image to a wide TV it won't look stretched. This was done with pre-DC systems too, like NiGHTS on Saturn, for example, since wide TVs were on sale in Japan in the early 90s. You'll have to use that mode that zooms in for letterboxed movies viewed from TV, or laserdisc...the one that only mattes 1 out of 50 movies correctly.
Now that I think about it, the 16:9 options may disappear from the option screen when the DC is booted in VGA mode. Yeah, I'm pretty sure they only work with interlaced signals.
Just so that are prepared, what you'll see on your LCD might not be the best the DC can do since LCD TVs tend to do a bad job at all sorts of slow-ass signal conversion for low/odd resolutions. Its either going to not even close to fill you screen, or it will be stretched. Hopefully you will be able to chose. It looks f*cking great on my CRT though, that's for sure.