Hi, I am too from australia.
Do not go for that graphics card, go for the 7870 XT, which is roughly around 40-50% faster I would think in most scenarios, for only a couple dollars more.http://www.mwave.com.au/product/sku-ab48036-sapphire_radeon_hd_7870_xt_edition_boost_2gb_gddr5
It also comes with two games which you can sell and thus get it less than the 660.
If you badly want the 660, the OC means its factory overclocked, in most cases the GPU is binned from a lot and the better OCing ones go in them. It will clock up to the overclock, yes, keep in mind the voltage is normally not pushed up, so basically no damage will be done to the card.
You'll run games pretty well, keep in mind GTA IV is a horribly optimized game, so don't really expect it to run it on ultra. Tomb Raider and hitman you'll see pretty good framerates, but not necessarily run it at ultra, I'm not too sure about that.
Your CPU and ram is great for gaming.