Microsoft has shed a little more light on what will define each edition of Windows 7. This time around they are doing a much better job than they did with Vista.
The maximum amount of RAM that will be usable will depend on which edition of Windows 7 is installed. Of course the 32-bit versions will all be limited to 4GB of memory, but for the 64-bit editions it is a bit different.
Windows 7 Home Basic and Starter editions will be limited to 8GB of memory. Home Premium will allow up to 16GB of memory and Professional, Enterprise, and Ultimate will allow up to 192GB of RAM. This shouldn't affect the average consumer, but enthusiasts will probably want to make sure they can utilize all of their memory should they be using more than 8GB.
AAC/H.264/MPEG-2 support will not be included in Windows 7 Home Basic and Starter editions but the others will have them. Home Basic and Starter users won't be totally out of luck as there will be add-ins made available although it is unclear at this time whether they will cost extra or be offered for free.