//Things gets more confusing, the more I get into it!!
Of course, Fermi is the newest and most advanced technology at the moment. The GPU accelerated rendering also. You should not expect popular and easy to access hardware solutions. Maybe in few months or a year. I am also researching for something affordable, productive and relatively easy to build, but I know a person has to be extra enthusiastic to sped his time in such way
I don't mind. I like to play with hardware.
I don't know are you reading the Randomcontrol's forums. They also have some hardware threads about these new GPUs. The hardware setup for Octane Station should be relatively similar.
About HP Z400: Keep in mind that upgrades will be problematic. All video card options are exceptionally weak for Octane rendering. These cards are simply not made for that. The productivity/price ratio is also the worst possible. The 475W PSU is so weak that it wont power properly even one GTX480... HP Z400 is good general purpose CAD station, but not really what we need for Octane.
I'll go back to my first selection. Personally I don't think I'll need more than 2 480s for what I do (product visualizations). I think it is still safe to run 2 480s on air cooling if the motherboard provides some extra space between the cards. That is one of the reasons I like MSI H55-GD65. Also for this setup it is still possible to use a single PSU and therefore some standard popular case.
If you don't feel comfortable about messing up with the parts, google for some assembly company. If you know what you want, they should be able to help you with the build. And don't forget to post some results here when it is ready