You should be fine with a 128 GB system drive. I have an 80 GB SSD with Windows 7 Home Premium 64, Cubase 6.5, Cubase 7, WaveLab Elements 7, WaveLab Elements 8, HALion Sonic, numerous other VSTs, etc. Space is a little tight, but everything is running smoothly. Whenever I do another full re-install I’ll go to a 120-128 range SSD just for some breathing room.
Another thing, if you have any 3rd party VSTs with sample content don’t install the samples to your system drive whenever possible.