A few notes.
Everything that has been said so far is true, but I want to add a little info.
If you pick up a SuperGrafx make sure you find one that comes with an AV cable. It uses a proprietary connector on the SuperGrafx end instead of the stand red/white/yellow composite connectors. You want one that comes with an AV cable because third-party/aftermarket ones suck and don't fit quite right. Finding [a good] one after buying the unit is probably going to be difficult. I can't remember offhand but the connector might be the same as the one on the Genesis. Someone else can probably clarify that. Although you probably don't want to use one of those model-1 Genesis cables anyway because they aren't stereo. And you
know you want to be playing Battle Ace in stereo.
Also, Japanese AC adapters. They all work fine in the US. In fact, it seems Nintendo has decided to cut costs by no longer producing US-localized AC adapters for the DS lite because the voltages used in the two countries are so close. They all seem to be coming with Japanese 100v-rated ACs, complete with Japanese text on the label and all.
I have an AV/S-Video selector box that takes any AV or S-Video signal and sends it to my TV via S-Video.
Unless you have a really high-quality switcher, any AV input into the switchbox that gets output to the TV in SVIDEO is going to be significantly downgraded. I found this out the hard way recently when I had my TurboDuo hooked up to a switcher via composite AV then output in SVIDEO to the TV. Joe explained it to me, I think the gist of it is that the luminance doesn't get separated from the composite video signal properly by the cheap switchbox. Whatever the technical reason, the result is a dark, fuzzy picture. It is much more noticable on lower-res consoles like the SuperGrafx as opposed to higher-res stuff like the GameCube or PS2. I don't know why this is, but I had a GameCube hooked up through the same switcher with composite and it didn't look quite as bad (but was still dark).
You have two ways around this. Buy a really high-end switcher, or buy a second really cheap switcher and use it only for your consoles with composite AV. Use the original only for SVIDEO consoles.
I only mention all this because I want you to get the best possible experience from your SuperGrafx.
BTW, keep us posted on your search for a SuperGrafx-- prices, etc. Don't buy the first listing you see on eBay, you will undoubtedly get ripped off.