The 70-400 is a surprising good zoom lens. The results far exceeded those of a Nikon 80-400 I used to have and at least equaled the quality of the Minolta 300/4 that got sold after using the 70-400. The down side is that it is big and heavy. Nevertheless, if I think that some reach beyond a 50 might be needed, then the 70-400 is on the A900, and the combo is on the big tripod. I finally tried the lens on the A55 a couple of weeks ago when an even tighter field of view was in order, and I was quite impressed with the results. When fully extended, there is some potential wobble, so I'd recommend your best long lens technique and extra shots for insurance.
Minolta's 200/2.8 is an excellent lens when you don't need the extra reach. It works great on both the A900 and A55.
Regards,
Rick