Hi there.
Basically Windows 7 Ultimate's storage requirements are 20GB for the 64-bit version and less for the 32-bit one. So you should be fine with 50GB of storage space as your C: drive, even considering the Windows updates. However you might need to manage your free space a bit better than people having a lot of storage capacity available for their C: drives. Make sure that you don't install any games on it and that you store personal files such as videos and photos on it, as they could take quite a lot of space.
Hope that helps.
Boogieman_WD