who has AT&T 2gb w/ tethering plan for their iphone?

i recently got an iphone and got the 200 mb plan but i am going to to change it to a 2gb plan and i was wondering. I know that tethering makes your iphone into wifi by using the 3g but what i don't get is will me using wifi from my iphone on my computer use up any of my 2gb of data more rapidly? Can i use the wifi for more than one device? can i still use my phone while it"s tethering? what is the range of the wifi? Sorry there is a lot of questions.
 
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