Dick,
We just updated to a GTN 650 on a IFR 206L3 last winter. We would have gone 750 if we could have made it fit, but it really wasn't in the cards. Thing is, there is only one advantage in my mind to the 750 - the bigger keyboard makes it much easier to enter intersection and airport identifiers.
We installed the Garmin FlightStream 210 so that we can talk to the 650 with our iPads and it's much better than just having a bigger panel mount unit. Being able to mount it in various places, or keep it in your lap works out very well. I think the new transponder (375???) gets you that now, even without the 210.
Not sure if you're aware, but you can upload and download flight plans from the iPad, so that gets around the issue of having to input identifiers on the smaller 650 unit. In our case, our avionics stack is on the far left of the cockpit, so using the iPad is also much nicer than having to lean across the cockpit to do everything on the GPS front panel.
Also, you get the synthetic vision on your iPad hooked to the Garmin via Bluetooth, so that's another great benefit. Not sure if ForeFlight works in OZ, but Garmin Pilot is another possibility - in my opinion not as good as ForeFlight, but might work non-USA if Foreflight doesn't. My buddy likes the Jep app, not sure whether that supports flight plan up/down load, though.
If you haven't looked into it as an option, I would highly recommend it.