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.
Main: Win 7 64 | Cubase 7.0.7 64 | WaveLab Elements 8.0.3 64 | i7-950 3.1 12GB | Focusrite PRO 40
Mobile: Win 7 64 | Cubase Elements 7.0.7 64 | Core2Duo 2.4 8GB | Tascam US-144MKII