OK - for Windows 7 x64:
RAM limits for Windows 7:
Basic - 8 GB
Home Premium - 16 GB
Professional, Enterprise, or Ultimate - 192 GB
So, IF you ever want to access more than 16 GB of memory in Windows 7, you need either Windows 7 Professional, Windows 7 Enterprise, or Windows 7 Ultimate.
8 GB memory is still OK for running Sonar - although when you get into complex, large projects, you may need to freeze things and be judicious with things loaded into memory.
16 GB should be a real good amount of memory, and IF you can get it, 32 GB is FABULOUS, but you should still be OK with either 8 GB or 16 GB.
I run with 32 GB and never ever have to worry about anything to do with memory.
Bob Bone