Given that the 83** were running 32-bit Intel XScale PXA272 312 MHz processor
The Bold (1) 9000 runs a 624 MHz processor which is the same as the 9700.
The Curve 8900 a 512 MHz processor
And the 8520 a 512MHz processor
All with 256mb of RAM. So the last 2 generations would all be suitable improvements for you. Just depends now on what you want to spend and what else you want/need it to have.
Well depending on budget I would say the world is your oyster, do you want/need 3G, if so its one of the Bolds. If not and EDGE is enough, you can choose from all 4.
See which is best tethered for you.
If you can do it, 9700 is the best out there really. As for carrier, I will leave you to check the other forums, and for people with experience to tell you. Being in the UK im useless for telling you that lol