It's no fun hauling two bodies around. Maybe consider a camera with swapping backs for color and BW if you plan on doing that alot. My 500C/M is like that and a treat to use for that purpose. Plus an added bonus, one filter size fits all the lenses I have for it. You could find yourself needing an 82, 77 and 67mm filters for 3 common lenses you might carry with you on the P6x7, for example.
Though not easy to find in the US and expensive the M* 300mm ED [IF] would be the 300mm to get. A friend had the other 300mm and could not get a sharp shot to save his life. Ultimately he had to get a rail system for a video camera and adapt it to the lens that provided the tripod mount and support for the front of the lens to resolve the problem.
Last edited by tuco; 09-12-2011 at 07:29 AM.