I've increased RAM to 256K, defragged my HD, dumped as many memory resident .dll's and .exe's as I can (and why in hell was Microsoft Intellipoint still loading, months after I had uninstalled it?), and I think things are a little better. A little. I've run Pinnacle's PCI bus settings optimization program.
None of this stuff made a huge difference, but all together, the Pinnacle DC10+ software is faster at playback, with fewer dropped frames and stuttering. It's definitely an improvement, it's just a long way from perfect.
I really expected the memory upgrade to make a large difference, and it didn't. The software still flogs the disk constantly. Maybe the video files are just too huge for a memory upgrade to make much difference.
Do I really need a faster processor/disk/motherboard, i.e., a new PC? Would Adobe Premier make any difference? Dunno. Not doing anything else for now, I'm going to ask around on rec.video.desktop and the Pinnacle forums to see what others epxeriences have been. Further upgrades in software and hardware will cost serious money, and I'll think for a bit before going that route.