I have a 120g vertex 2 and I use about 35GB without games installed, pagefile on 8GB, hibernation on 6GB, and the most essential program for works such as photoshop, acrobat, firefox, firewall, antivirus and spyware remover installed. If you disable hibernation and have less ram so that page file is smaller say 4GB or lower/disable pagefile, you will end up using about 21-25GB, so I think 30GB is doable but IMO you are cutting it very thin.
30GB is pretty marginal. Yes, a basic Windows install only takes around 12GB or so, but it will go up substantially when you add the pagefile and hibernation file. Additional programs will of course add to that. And over time restore points, log files and other growth will cause your free space to slowly disappear. You'll likely end up constantly having to look for ways to pare down unwanted files in order to recover space.