Thanks guys. All very helpful advice. I am looking at the Mac Mini now. I had thought by the name they were an entry level machine ... how wrong I was. They compete well against the price of a Hackintosh and without the pitfalls.
Jeff, you are right about the Hackintosh requiring constant fiddling. That would get old very quickly for me.
You also raised an important point about the Retina's non-upgradable non-repairable innards. That is a concern. You would want extended warranty and thats over $400 in Australia. It's the screen that seduces me ... Oooh look, shiny things!

So now I'm thinking Mac Mini 2.5ghz for $869 and upgrading the RAM and drives myself. 16g OWC ram and 240g mercury extreme pro SSD. $1386 total (everything's more expensive in Australia

).
Re: my G5 Tower. It served me well. I did upgrade a lot inside and I was running OS10.5.8, so it performed quite well and was incredibly reliable, but it is time to start new. Even if it is only a power supply fault, actually getting to the PS without damaging the CPUs and mobo requires a service manual and they don't exist anymore. Presuming I could achieve that, I may discover that it the fault is in the mono or one of the CPUs.