I think there is only one current model?
The difference is the hard drives.
You can get one with a 4GB built in drive and then add additional memory (such as a 320gb) or buy a 250GB console (not sure if that's built in memory or also expandable.
I got the 4Gb and a 320GB hard drive in November (hence the lack of knowledge) from Makro on a good bundle deal (with games and extra controllers)
You can pick them up at pretty much any large shop (Game, Incredible etc) or off Bid or Buy. If you're looking for the Kinect as well, that usually comes with the 4GB console, or you can buy it seperate- around R2000.
I paid R4000 for mine, 5 new release games, console, 2 controllers, 320GB HDD and media remote at Makro.