There was only one problem.
I had 3 different authoring packages.
The machine came with Sonic Solutions Digital Media Studio V7 to support the built-in CD burner. When I upgraded the video, the card came with Cyberlink DVD Solution. The new burner came with Nero Vision Express 6.
So I had the Cyberlink, I like the Cyberlink, I decided to try the Cyberlink PowerDirector. It's got a great interface. It's easy to shuttle through a video file, drop in chapter marks, set chapter thumbnails, and prep your DVD. Unfortunatly, there are major disconnects in their transcoding system, so it couldn't actually convert the DivX video file to MPEG-2 for the DVD. That's what "There is no common media type between these pins" (the stupid error message it gave) means. Scrap that.
I had the Nero. I don't really like Nero, and I hadn't installed it, but I figured it was worth a chance. The authoring interface is funky (as in "stinks like soggy swamp feet," not as in "has a good beat"). Chapter marking is twitchy; there isn't a lot of fine control. Setting chapter thumbnails happens during menu editing, not during chapter marking, and the level of control is even less friendly there. The DVD menu templates included with the software are pretty bizarre and can be difficult to read. The transcoding works like a champ, though, and can be configured (if you're willing to burn the processor time) to make the most of your image quality. I also found the free patch to Nero 6 Ultra Edition; it has a few features that improve on the interface.
I forgot about the Sonic. I had never used it (I haven't used that machine to burn a data disc, and I burn my audio discs with iTunes). It had Sonic myDVD, though, so it was worth a try. The interface was even funkier than Nero. I never did figure out how to set chapter thumbnails. The menu templates were much prettier than the Nero layouts, but were even less flexible. I couldn't figure out how to create a DVD with a single video file that only showed the chapter menu, and whenever I changed the first chapter title, the chapter menu title or the file title the whole thing changed. I didn't even bother attempting to burn a disc.
For the moment I've scrapped all the Cyberlink software except PowerDVD (I still need a DVD codec, and it's a good one). I scrapped all the Sonic software; I couldn't find anything good in there to keep.
I installed all of the Nero software and patched up to Nero 6.6 Ultra Edition (a bit surprising, most of the other packages don't offer free upgrades for OEM versions). I'll download the free template packages when I get home; perhaps they'll include some less-ugly menu templates. The Nero 7 Ultra Edition is also tempting, but that costs, and I don't see any reason right now to pay for more.