It sounds like one of those software updates was a firmware update that failed. Try booting to the original USB drive and see if it'll just start up ok. If it starts up just fine from that, then your problem is in your system, not hardware. If it does the same behavior, then it's probably firmware, & I think to correct a failed firmware update, you must be able to boot to an installed OS on a drive with the firmware updater on it, so this may require a trip to an Apple store to resolve.
You should be able to boot from your Snow Leopard DVD disk, and before going through the installation, see a menu option at the top of the screen for "reset password." That said, are you certain that g5 can run 10.6? I'm pretty sure they're topped out at 10.5.8 and 10.6 will refuse to install.