Visiting USA: PAYG data plan?

[fk123]

I will be visiting the USA for a couple of weeks in late summer.

I am now looking for a pay-as-you-go (a.k.a. "prepaid") SIM card that offers me a certain amount of data that I can use. I am guessing that I will use up to 500 MB over the air.

Do you have any suggestions about a reliable PAYG data plan?
Where could I find more information? The AT&T and Sprint websites are a real horror to navigate through. :-)

[fk123]

http://www.wireless.att.com/cell-phone-service/go-phones/gophone-options.jsp?wtSlotClick=1-0025SG-0-2&WT.svl=calltoaction

So, at AT&T, it is $20 for 100 MB on a GoPhone plan. That's 20¢ per MB, which is OK for prepaid.

Virgin Mobile offers 50 MB for $20.
http://www.virginmobileusa.com/rates/minute.do

With T-Mobile and Sprint, I couldn't find a prepaid data plan.