If you delete your Bootcamp partition, presumably you will have a Windows license freed up. Why not run Windows 7 in VirtualBox under Mac OS?
Or if you are willing to pay for Parallels (or VMWare Fusion), you could import in your Bootcamp setup into a virtual disk file and then nuke the Bootcamp setup. Virtualbox, to my knowledge, does not offer this feature. For Virtualbox, you would just have to create a virtual disk file from a scratch installation of Windows.
Of course, there will be some limitations. At a minimum, you will need to keep the Bootcamp partition in place while creating the virtual disk file from it. This means that you need to have enough disk space on your internal drive (or an external drive) to fit the created virtual disk file. So, if the Bootcamp partition takes up about half of the internal drive, then you may not have enough room on the internal drive to import it into a virtual disk file unless you use an external drive.
BTW it can't be a Mac G5 you have - they don't run Sierra (latest version with PPC support is 10.5).
Not that it matters, but nor would a Mac with a G5 processor run Bootcamp or Virtualbox. Both require an Intel processor in the Mac. If it was a G5 processor, then the only option would have been Microsoft's emulator VirtualPC (originally created by Connectix and bought by Microsoft) or some other emulator (i.e. a program that could "emulate" an Intel processor...only know for sure of Virtual PC, but there might have been others), which was SLOOOOOOOOOOW because of the emulation required.