iPhone 4 data plan how does it work?

if you use it alot get the 2gb plan if you go over 2 gb its 10 per gb over

Does it affect the 3G or what

Of course. You're using your wireless providers 3G network for data.

2GB is a lot of data, and the data plan is cheaper than the old unlimited data plan. If you don't know how much data you expect to use, then you are probably better off getting 2GB, then after a couple of months, evaluate your useage and determine if you will be OK to switch to 200MB

u can always use Wi-Fi and some app, bigger than 20 MB will not download anyway over 3G. Wi-Fi does not use your data plan.
I would say start with the $ 15, for 200 MB and monitor your usage.

I would disagree with starting small. If you get the 200MB for 15 and use 800MB you will be charges $60. If had got the 2 GB you would only pay $25. Your better off getting more, then checking your useage and ajusting downward than paying an overage 2, 3, 4, 5, times, etc...

If you got the 200MB and used 2GB AT&T would charge you for the 200 ten times. Your better to start high and ajust lower.