You cant install ios 5 yet on a 3GS with the ipad bb.
You have to wait till there's a tool that would create a custom 5.0 beta ipsw.
And then ultrasnow will not be compatible with the firmware version and will need to be updated to work.
So in other words, stay where you are. Wait for the official JB release, wait for ultrasnow to be updated and stay away from betas if you're not a developer.