Well my Win7 x64 installation occupies 20Gb, but I have a fair few programs installed which require entries in my users folder - but the majority of my programs are installed in a different partition, as are my data files and my Page file. So, it depends on which and how many programs you are planning to install and if you keep your data on the same partition as your OS (something I do not recommend). I would say a minimum of 20Gb, but depending on how you intend to set your system up, I would go for 50Gb to be comfortable - if you have the space.