Guys, I think I may have solved it. I believe that all SD content is 4:3, including games, but can be in 16:9. This is done by anamorphic widescreen, so anything SD that says it's widescreen is anamorphic, not really true 16:9, but it should still be used.
Luckily my projector has a clear aspect ratio setting called anamorphic, which I'll use only for games that have a widescreen capability. If it doesn't, leave it at 4:3 I guess. Have I got all that right?