Mark,
From the point of view of ArchiCAD, if you're thinking of using AC versions older than v.11 then you might want to look at Pro (which includes XP-mode). Other than that, Home Premium works perfectly well.
Pro also gives you the ability to join a server domain (used by large organisations), encrypt your file system, run a remote desktop server.
All I can suggest is that you go with 64-bit if the hardware supports it.
Good luck!
