I have had an opportunity this week to try both the Nikon D300 and the D3, and I have decided that the D3 will work best for me since it has a full frame sensor to allow very wide angle work, and I think the AF system is a little quicker and more foolproof for action / wildlife shots. I also really like to double CF card slots for in camera back ups, and the weather proof seals. I have "lost" a number of good shots over the years due to wind blur and lack of ability to hand hold the H3D in low light, so I think this will help be broaden my creative opportunities. It will not be a replacement for the H3D but will be a tool for uses the H3D is not suited for - low light handheld, action / wildlife, windy conditions, super wide angle, super telephoto. I do think a full frame sensor has an edge over the smaller sensors in regards image quality and noise, especially when I need to make a 100" long blow up (which I have had to do a few times this last month) . I will be photographing wildflowers in the Sierra foothills this coming week and will post some shots with the D3 after that trip. I am then off to Hawaii, where I will try it out underwater in an Ewa Marine Bag (yes I'm Crazy, but I insure all my gear in case of the worst.....). The underwater housings for the D3 run $6K, so that is out of my range unless I get a sweet photo assignment from a dive magazine...highly unlikely given all the great underwater photogs out there. The live view feature should be a good tool for underwater shots of turtles and colorful fish.
I think the D300 is a great second body to have in addition to a D3 - it is a good backup and nice to have a different lens on when doing a photo shoot. I found some good info on the D3 on Moose Peterson's site. Alot of his info also applies to the D300.
As far as lenses, I am going with the awesome new 14-24 (one of the main reasons for getting the full frame D3), and the 24-70 f2.8. I also got a lower priced 70-300 VR lens, which would need to be upgraded prior to any serious wildlife photography to the 200-400 VR.