Hello ...well technically you can upgrade to 5.0. It must be custom firmware to preserve your baseband. I used sn0wbreeze and then ultrasn0w. I have a 3GS, 5.13.04 old bootrom too.
The problem is that Apple stopped signing 5.0 so you can't upgrade to 5.0.
Do not go to 5.0.1 that is unlockable.