Your idea is demonstrably untrue based on the fact that people have been doing realtime Firewire NTSC video input for years, and that's certainly FW400. I'll admit that it's near the threshhold, but not there.

NTSC is 720x540, tops, but if we're talking about setting it up as a Windows display, let's assume the worst since it's not going to wander back and forth. The framerate is 29.97fps. Assuming 32bpp, that comes to a little under 356Mbps, which is well less than Firewire's 400Mbps.

PAL maxes out its resolutions at 768x576 and has a framerate of 25fps. Again assuming 32bpp, that's 337.5Mbps.

Both of those numbers are also under USB2's max bandwidth of 480Mbps, though I wouldn't be surprised if USB2's overhead put them just over. Still, if you reduce that 32bpp to 24, you get 267Mbps and 253Mbps, which are both well under both limits.

I'll admit that (1080i) HDTV resolutions would be too great, even for FireWire800, at a little under 1.2Gbps, best case. 720p resolutions could probably fit in FireWire800, though.