MacFamily Tree that you are running now.
Any new iMac will arrive with OS X 10.7.x (Lion) installed and as such may not be capable of running your version of MacFamily Tree because Lion does not support applications that were written for the PPC powered Mac.
Version 4.5 is long in the tooth so an upgrade to the current version, 6.x, would probably be your first move. It's available from the Synium web site for $25.00
As to moving your data, when your new iMac arrives and you first start it up, you will be given the chance to move all your data from the old machine to the new one. Do that before you do anything else.
You will need a Firewire 400 to Firewire 800 cable or a regular ethernet cable to join both machines together.
On completion, you will have everything moved, including your MacFamily Tree data but the application may not run on Lion. Now go to the Synium website and purchase version 6.x of MacFamily Tree, download it and run it.
Import your old data into the new version and you will be all set.
FYI, there is a whole section on these CNET Forums that are exclusively Mac. Pop over there and check it out.
I am buying an iMac and want to transfer all the data to it.