Purchasing iWork for MacBook Pro and Mini Mac

If I have a MacBook Pro (older 2006) and a Mini Mac, do I need to purchase iwork for each? Additionally, would I need to purchase pages '09 for each as well?

Also, I'm running Lion on my Mini Mac and Snow Leopard on my MacBook Pro. Will this affect my purchases?

Apologies if this is pretty common knowledge. I'm still getting to know all this.

Thanks!!