You will probably get a bluescreen when Windows starts due to the difference in Windows driver between the 5i and 6i.
ie, Windows will be looking to boot from the 5i and its no longer there.
I have had success before by installing the driver for the 6i while the disks are still in the old server, THEN swapping them over.
Your mileage may vary though, and I have had this not work before also. In this case you can reinstall Windows over the top of itself, then install the latest / last PSP for the G4 once Windows is up and running.
You wont lose any data but WILL lose desktop settings, shortcuts, may have to reinstall apps due to registry entries being changed etc...
Bare metal backup normally means taking an image of the whole server, rather than just a files / folders backup.
Although I think you'd still encounter the same problem above with the drivers when you restored the image to the other server...