That mainboard can fit up to two dual-slot cards and three single-slot cards. You have chosen a PSU with 6 native PCI-e power connectors, so unless you do molex to PCI-e that will be your limiting factor.
My advice would be to get 5 of the single-slot MSI GTS 450
and rock on from there. You could replace one of them with a GTX 460 for a bit more PPD, but I don't think it's worth it. However, if you get one of these
plus 4 GTS 450, you will be absolutely maxed out.
Tats has a good point: What is your cooling solution going to be?
For a case: Test bench is probably your best bet. Make sure it's in a well-ventilated space and that you clean it at least weekly.
EDIT: I didn't even consider the PCI-e extenders. If you go that route, then you have lots more options. What's the budget?