Use redsn0w to create a custom IPSW to update to. It'll allow you to preserve your Baseband while still updating to the latest iOS version. Do you know what SHSHs you have available for your 3GS? If not, download Tiny Umbrella from ih8snow.com and see if you have SHSHs for 5.0.1. That'll help since there's an Untethered JB for 5.0.1.