nah, the Nine Hundred should have ample space for everything (granted, the video card might be a tight fit).
as for the OS, 7 Home Premium should work just fine as its designed with all of that in mind. the main selling points of Pro is that it gives the option of connecting to an external network via domains and an XP mode that actually uses XP running as a virtual OS (w/o 3D graphics accelleration, btw). unless you need either of those features, stick with Home.
otherwise, it sounds like a great build. good luck.
See the struggle of the faithless lot as they negate their time
How low to sink to the depths of their frame of mind