Using iMac as a second display

To answer your question : Yes, in display preference when you connect a second monitor there is a place to "arrange" the virtual position of the different display and you will see that one of the display have a "white bar" on top, you can drag and drop this white bar on the display of your choice and this display will then be the "main display" (with the white menu bar on top and dock at the bottom).

About your question, an iMac G6 doesn't exist. the "G3 G4 G5" were brand of PowerPC CPU that Apple doesn't use since they passed to the same x86 architecture as the PC world, Intel CPUs have been used since then.