In the pro model, it says Windows 7 supports domain join. But on Vista I currently use domains... I think. I'm no network pro, but I managed to get a spare computer running Ubuntu + webmin to act as a NAS. I have a few PCs, one which will have Windows 7, another with Vista, and another with XP. Besides using a Ubuntu NAS, I want to be able to share files outside of the "My Documents/Pics/Music" etc. folders, and I want to be able to play LAN games.
Does domain joining affect any of this? I currently can do this on XP Pro and Vista Professional. Do I need Windows 7 Professional to do this, or is Windows 7 Home Premium enough?
A NAS is not a Windows Domain. A Windows Domain is a server that runs Active Directory and which holds all of your user accounts along with all of the other computers that trust it for authentication.
You don't need a Windows Domain to share files and play LAN games, Home Premium should work fine for those purposes. You might want to take a look at this blog entry that describes the differences between the various editions of Windows 7.