For me it came down to price. Four 5830's were ~$450 USD. Two 6990's were over $1400 USD. That's a big jump. Your setup is certainly cool though.
No argument there - 5830's are definitely a better "bang for your buck".
I actually need to pick up another PSU soon so I can run my new cards. My "desktop" machine has a Corsair 750w in it now - so I'll probably pick up another AX1200. (I'd only be able to run 1 of the new cards as it is now)