well I duno much about the southern California are as I'm from Arkansas but I do know allot of people use tmobile. With the iPhone. Would u like to stick with virgin? If so it maybe possible. Anyways...
when u chose your service you will want to get the iPhone download ziPhone " the current one step jailbreaking and unlocking process" and and run the program and presto your iPhone will work with your picked carrier.
The program alows the iPhone to run on any sim card you use. When u unlock the iPhone it makes the iPhone open to other carrier sim cards basically. If you wanted to know more about it in detail I would suggest sending a pm to themanofthedark. He is the house onlocker around here with exstencive knowledge on the subject.
That will be fine, but you will be charged $.01/KB everytime you check the weather, surf youtube, check stocks, Google maps, or surf Safari. I typically use 25 - 30 MB/month and that's just what I do on EDGE. Most of my surfing is on WIFI at work and at home. You could possible use 60MB of data in a month that would cost you $60 alone. If you're never going to use the apps you'll be okay, but if you want to use the features that make the iPhone unique, then you need a data plan.