As far as I know, there are no ISPs that will support Linux officially. A few reasons being is that it takes too much effort to maintain reliable support stuff, almost all modems uses winmodems which generally do not work with Linux, too many distributions to keep following, etc. May be you can check with local LUG to find out if there is any in your area. The last time I checked, netzero works but it may no longer the case now. Try to find out what authentication they use <good-luck-there> (netzero used to use CHAP).
If you can get DSL, it's always better to use it. Only a few extra bucks a month and it's well worth it. If your location is too far from CO, then you have very few options. Check out mailing list at http://www.lugod.org/ . They have few members who live in NorCal such as in Davis. They might know which ISPs are okay.