A strong G connection will allow you to stream HD. That said, the quality of your wireless connection is entirely dependent on environmental factors in your location.
I watch Netflix movies in HD and access MPEG-2 HD on my NAS through my TiVo box, which is connected wirelessly to my network. I started with a G adapter and changed to an N adapter simply because someone gave me one and I noticed no improvement in performance for the simple fact that my G connection was robust enough to begin with. The access point sits directly below the TiVo box, one story down so it gets a solid connection. It's extremely rare that there are streaming issues and when there are, they are momentary.
If you go with N spec, you'll likely be in better shape with improved bandwidth and range. If you have any doubts as to range issues, go with N.
Is this for mobile purposes or a fixed set up? My kids watch Netflix and access the NAS through their laptops in the house. They know there are spots where they don't get a good connection and they simply avoid them. If you need seamless streaming you may need multiple access points if you live in a big house or an older house with plaster lathe that will interfere with the signal.
And yeah, of course ethernet is the best option from a reliability standpoint because it is not prone to the interference or range issues that wireless is. If you can do what you need to do with ethernet, go for it.