Okay, I stand by my recent complaints about NVIDIA's linux support, but I now have some additional perspective. It occurred to me that I should try swapping the 5700 Ultra in atlas with the 9800 Pro in parabellum. (That way, I could justify buying a new 6600 GT for parabellum--but I'll probably do it anyway.)
The best I could do was this, using the latest (8.10.19) proprietary drivers. The garbage on the bottom of the screen blinked and moved, indicating just how wrong the framebuffer was. The cursor was movable, but running X programs or trying to kill X was not possible. Too bad. I was excited when ATI's setup program asked what should be my primary display, as that is a major complaint I have with NVIDIA's TwinView (you cannot set a primary display in the single framebuffer mode... and it defaults to the *CRT*!)
Ah well, it's back to my 5700 for now. During my troubleshooting, I changed my AGP driving strength to EA (it defaulted to FF which seemed wrong to me) and forced AGP 2x mode. Perhaps that will make a difference on my NVIDIA crashes. I do not feel like testing this now.