SSD and Windows 7 help?

I want to get an SSD solely for the purpose of installing the OS into it (Windows 7) so that the boot up time of my computer is much faster. How big would I need the SSD to be? I assumed that 50GB would be fairly plenty, but was told otherwise by other ppl that I need at least 200GB for it. That sounds like way too much to me, as an OS doesn't need that much space. Can anyone help me out?