Hi,yes you can tether your 8320 without a Blackberry data plan,after a lot of Googling I found out how to do it for our daughter who uses her 8100 with just a generic data plan.The steps I followed were:-
1,Follow the instructions from Anworm to install a "browser via tcp" service book,this will allow your 8320 browser to work without a Blackberry data plan and will turn your BB into a useable modem.Instructions here.
2,Follow any one of a squillion sets of instructions to tether your BB to your Laptop.I actually used the guide over at Berry 365 which can be found here.
You will now be able to use your 8320 as a modem amd also because you have enabled your BB browser you will be able to carry out downloads and OTA software installs etc.Also if you want to you will now be able to install one of the several third party push email apps and have your email pushed to your 8320.Our daughter uses Shangmail which I must say is excellent for a free app,it polls quicker than Blackberry Email and intergrates completely with your phone so that you can assign it tones etc.
I hope this helps.