RE: Nikon D300
If you own a DSLR now and are familiar with all its possible setting, ISO, continuious focusing, Apeture and Shutter priority, I think you'll do fine with a D300. It is a wonderful camera, no I don't have one I'm a Canon shooter, but Nikon's new cameras are amazing, the D3 and D700 are masterpieces of engineering. If you struggle with a point and shoot you might stay away but, I think you'd love a D300.
IMHO.