I believe that the way to target the widest mac audience with a 'first out of the gate' app would be to build a Tiger (OS 10.4) 'Universal Binary'. In most cases, anybody with OS 10.4 (Tiger) could run the app whether they have an older Motorola PPC G4, G5, or newer Intel-chipped machine. And typically, a Leopard (OS 10.5) Intel Mac user could still run a Universal binary built for 10.4.