DVCPRO 50 is pretty much 8-bit uncompressed. You'd be hard pressed to get that working properly on FW400 drives. I have gotten 8-bit and one stream of 10-bit uncompressed to work on my FW800 G-Raids. And currently I am using the G-Raids to edit DVCPRO HD footage.
And yes, you need at least FW800 for DVCPRO HD. It will not work with FW 400 drives...throughput requirements are too high. I get all my info from
http://www.hdforindies.com.
Shane
Alokut Productions
http://www.lfhd.net