first off why would you get a ham radio when your CB works fine?

-power output
Mobile HAM radios typically put out 35-75 watts of power. Your CB is limited to 4 watts legally. Does this mean you can reach ten times farther with a Ham radio? No, but 3-5 times further would be a closer answer with all things equal. Yes Ham radios can reach insane distances in favorable conditions and extremely far using repeters

-Repeters
There is a network of Ham radio stations on mountain tops all over the country/world.
An amateur radio repeater is an electronic device that receives a weak or low-level amateur radio signal and retransmits it at a higher level or higher power, so that the signal can cover longer distances without degradation
signals are also relayed back to you so you can hear your contacts voice even though there may be mountains and large distances between you. some repeaters even allow you connect to phone lines(somehow). the network of repeaters is quite good because they often share the same sites as other radio towers. some areas in death valley are only covered by repeaters and no cell coverage
kinda interesting but here is a small 70cm home brew repeater near me. its actually hooked up to the internet to.
http://www.youtube.com/user/KR1IS#p/u/1/ttNc8C8bzMo

now onto frequencys.

the basics
70 centimeter(aka 440 aka 446 aka UHF) this would be the second most popular for mobile
2 meter (aka VHF aka 146) - this is the main freq range for mobile
6 meter - not really used for mobile
10 meter - not really used for mobile
11 meter - hey thats good old CB!

so how do you determine what frequency bands you want to be able to use? first off get what your friends will have. for offroad, i havnt read about anyone not using at least 2 meter. now any secondary freqs you might need depends on who else you need to talk to. mainly of interest is going to be 70cm repeaters. for some reason 70cm repeaters are plentiful in cali and dwarf the amount of 2m repeaters in urban citys of Arizona. see the attached google earth file to find repeaters in the areas you go.

thats about it. i havnt learned much but these first steps of knowledge might be helpful to someone.