Like the others, I have nothing but good things to say about Win 7 x64 - I went right away when it was released, upgrading from the mess that was Vista and Win 7 has been rock solid.
As far as particular versions of Win 7, Ultimate would be a good choice as it allows access to a limit of 192 GB or physical RAM. While you may never use it, it's nice to have the ability to upgrade your system with RAM especially if you use large sample libraries.
I always recommend that people get at least the Home Premium level of Win 7 as it allows access to 16 GB maximum - Home Basic only allows up 8 GB, and Starter only 2GB.
I currently have 8 GB of RAM, but I just got a new MB that allows more so I think I'm going to upgrade to 16 GB soon. That combined with my new Quad core processor should meet my needs for at least a few years I hope.