I have been hacked. what do I do?!

So I believe I've been recently hacked by a company named abita technologies. I'll list what happened:

Downloaded a song, a popup showed up on my macbook pro saying that its detected a virus, couldn't exit out of safari, I panicked and called the hotline that pop up provided me with, i talked to one of the associates through the hotline, gave them my mac's serial number, i gave them permission to authorize my computer so that they can "fix" my virus", paid them $300 to fix it, told me to wait an 1 hour- 1:30 hour. Things I've done so far: changed all of my social networking passwords, canceled all of my credit/debit cards, and I filed a complaint to the IC3, as of right now I disconnected my modem so that no device is in contact with my modem, and I erased my disk so it's at the factory settings right now. I have not reinstalled the OSX software yet since I have to use the Internet to install it. I have a few questions.

1) do I have to get a new macbook since i gave them my serial number?

2) is it safe to turn back on my modem? (left off the whole day)

3) will I have a new IP address once I turn back my modem on?

4) what are other suggestions I should do to protect myself and my identity?

Any other recommendations are welcome. Thank you.