How does one work out what speed fan will be sufficient?
(The case in question has a 120mm intake and a 120mm exhaust fan)
And I'm in Melbourne, Australia - so yes we do get rather hot summers..
Theres no real way to "work out" what fan you will need, but you can get a pretty good idea.
The CPU and GPU are similar to mine in heat output. I use a Scythe Slipstream 1200rpm exhaust and a 92mm Scythe kama as intake (running at 7v). This provides ample cooling. When not stressing the CPU i can turn down my exhuast to 5v (with the zalman fanmate, very handy tool), if im gaming or encoding i can turn it up to full speed and dont have to worry.
In your system i could say 1200rpm exhaust. For intake you really just need it to direct the air that is being sucked in anyway over the HDD and towards the GPU. Go for an 800rpm here.