Windows 7 uses less RAM and disk space than Vista. The installation was done on a test machine running Windows XP SP3, the virtual machine was configured with 512MB of RAM. And the result, the x64 edition of Windows 7 Ultimate running on just over 200MB of RAM and less than 9 GB of disk space in this VM.
Why does Windows 7 (download) use so much less disk space than Vista? A very small amount of the savings (much less than I expected) is in program code. The biggest savings is from the preallocated volume shadow storage space, which holds System Restore points and backs up files via the Previous Versions feature. On my Vista virtual machine, this feature was using more than 4.6 GB of disk space. Under Windows 7, the system reserved less than 400 MB.