The $20/mo puts you on Cingular WAP servers. Slow as f00k. The $40/mo puts you on the isp.cingular (or whatever they changed the names too now that it is ATT again.)
And just so you know, dont call up ATT for help, as tethering is a blatent violation of the TSA.
And if you mean have the phone connect via GPRS/3G whatever, then yes you will be charged. If you mean having it dial a phone number like dial-up, then you will just be charged regular minutes.