How to tell if my iPhone 4 can work with straight talk or even virgin mobile?

Okay so I bought this iPhone 4 from my friend yesterday. It isn't jailbroken, and I kinda don't know if jailbreaking is a good idea. Anyway, this iPhone has a SIM slot on the side. It came originally with an AT&T SIM, and worked with my old AT&T GoPhone SIM that I had cut. Now, I don't know if this feature means I can put any cell phone carrier's SIM in there and it work, or what. I'm unsure of Apple in general.lol.
If anyone could give me advice on this this would be great. I had Virgin Mobile before I bought this phone, and I'd like to stay with Virgin Mobile. I mean 35 dollars for unlimited text and web and 300 minutes of talk? I think that's pretty great and it always has worked with my budget. I don't want to use At&t because their prepaid data plans are stupid. Just stupid. My virgin mobile phone before didn't have a SIM in it, so I'd have to order a new SIM for whichever carrier I decide upon. Straight Talk is another option, too.
I just really need some advice here, Apple and I aren't too familiar, so it's quite a puzzling process for me.
I'll appreciate answers greatly.

Sorry to break this to you, but Virgin Mobile used CDMA towers. Since the iPhone is originally from AT&T, they use GSM towers which means no CDMA carrier can be used on the iPhone. Fortunately, Straight Talk does provide GSM service and you can buy a micro SIM card at StraigjtTalk.com And to note, Jailbreaking isn't bad but it can slow down your iPhone.

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