The simplest way to connect your laptop to your mixer would be with an external soundcard I would think, In that case you would normally connect your keyboard to your soundcard with a midi cable, then your soundcard would connect to your laptop with a USB cable and your mixer/monitors would connect directly to your soundcard as well.

Hugh Sung explains it quite nicely here http://www.youtube.com/watch?v=gf1t1qs0yTs

I play in a band and a duo, and would eventually like to use Pianoteq at our live gigs. 

At the moment I have a KORG sg-proX keyboard and a Yamaha p-120.  I also have an iMac running OS X with a 2 GHz intel core 2 duo with 1 GB RAM.  I am wondering how in the world I get the KORG connected to the iMac?  Once the piano is connected to the iMac, how would I then get it fed into the bands mixing board?

If I were to purchase Pianoteq, I would have a little over US$ 300 left to purchase the other necessary pieces.  So exactly what would I need for a good basic system, and is it possible within my price range, and if possible, do you have any suggestions for what products are best?

Possibly related, eventually I would like to buy a laptop for convenience reasons. Can I transfer pianoteq to a new computer?