Looks like you'd have to Upgrade at least the OS...
Macs with Mac OS X v10.6.6 or later that were produced before those referenced in the sections above can use FaceTime for Mac. Calls will be sent and received using standard (non-HD) video.
Snow Leopard/10.6.x Requirements...
* Mac computer with an Intel processor
* 1GB of memory (I say 4GB at least, more if you can afford it)
* 5GB of available disk space (I say 30GB at least)
* DVD drive for installation
* Some features require a compatible Internet service provider; fees may apply.
* Some features require Apple’s MobileMe service; fees and terms apply.
Which apps work with Mac OS X 10.6?...
It's been pulled from the online store, so you have to call Apple to buy it.