For the memory you want all 6 slots to be filled with the same memory. So, for 12GB total you'd want 6 x 2GB sticks.
The next step up would be 6 x 4GB for 24GB total.
Don't rush on that.... I'll get you some part numbers from HP and then you can use those to find good buys used on eBay. Not sure how easy it is to buy off eBay for you Aussies.
The 2GB sticks in particular are very reasonable now and honestly for many of us more than 12GB is not needed. I'm running one of my Z600s with 24GB and my Z400 with 12GB and I can't tell the difference. I'm not doing nuclear engineering here.....
