3g stuff in the USA

Ahh with the risk of sounding like I need to do some research (I have, I have! I'm no good with Google...)
Well I'm going to the USA in August, and I have an iPhone 4s which I'd like to have some 3G data when i'm there. Just need to use the Internet, I won't need text or calling. I'll be there for a few months, so can't have a contract or anything (if that applies to non citizens).
Can anyone recommend me a provider?
Cheapest would be good. I'll need something monthly from around 200mb onwards :)
Thankyou!

see....MAXROAM.COM /
is your phone unlocked ?

If your phone's unlocked you might be able to go month with some carriers. T-mobile & AT&T are GSM, Sprint & Verizon are CDMA. The last three are iPhone carriers - I have AT&T with a 4S. You may have to put up a deposit, though, so make sure to clarify that first thing!