Okay, so I downloaded a Knoppix CD overnight, only to realize that I didn't have any 80min CDs to burn it to. So I had to burn it to a 74min one and hope it got enough data to do something. As it turns out, just. It got to X Windows in a definitely different resolution and timing. Of course, it was using VESA modes, but I can't imagine that the Windows installer wouldn't be at that point in the install, other than simply a different resolution. So it looks like it's not the video card and it's not AGP compatibility; it's just Windows being screwy.

BTW, Knoppix looks incredibly well done. I'll have to reburn it on an 80min CD and play with it.