Most cameras only offer FW-400. I do not know of any, that offer FW-800, but there could be some.
The speed of FW-800 is easily benchmarked, when one is editing to/from. I do not know how those two might differ with Capture, should FW-800 be available. With FW-400, Capture is pretty much in RT. I do not think that FW-800 would be much faster, as one still has the speed of the tape as the base. I just do not have any FW-800 Capture experience.
And the 400 is more than adequate for capturing full quality DV-AVI video.
I do want to add that I have been very pleased with FW-800 for my external HDD's. I can easily edit to/from those with FW-800, and then can just transport my entire Project, Assets and Scratch Disks between different computers. Note: if one does this, set the drive letter of each external in the OS of each computer and put a label on each external, so you know what it will be. I stared with Z:\ and moved backward, so that each external is seen as the same drive letter on all computers.
I had tried editing to/from USB 2.0's, and had major issues, plus they were horribly slow. Moved to FW-400, and had no issues, but they were still too slow for me. When FW-800 came out, I tried again and loved the results. If one used eSATA, it would be even faster.