I currently have an LG Vu Plus 3G phone on AT&T, with an unlimited calling, texting, and data plan. But I want to use an Android phone instead. I found a phone I like on Amazon - it's an unlocked Samsung with Android, and it was originally a T-Mobile phone. If I purchase this unlocked Samsung phone and put the SIM card from my AT&T phone into it, will there be any changes to my phone plan? Will I have to tell AT&T that I'm using a different phone model? Will my mobile plan change at all in terms of cost or features? I know absolutely nothing about phones and what exactly is stored on the SIM card.
I guess here are my questions:

1. If I put my SIM card from my AT&T (non-smart)phone into an unlocked smartphone with Android that was originally used with T-Mobile, will it work - will I be able to make and receive calls and texts, access the internet, and use the Android features like the Marketplace?
2. Will AT&T need to be notified if I put the SIM card into a different phone? Will they have any problem with this?
3. Will the unlimited data/text/calling plan I already have with AT&T carry over if I start using an unlocked phone? Both phones are 3G if that makes any difference (I'm still not really sure what 3G is)

Thanks, and I'm sorry if any of these are stupid questions - like I said, I know next to nothing about phones and much of the terminology is confusing to me.