1640/2285 is .71 megahash per dollar.
An efficient 5830 rig could get 1.36 megahash per dollar so it would cost about 1,205 dollars for the same speed.
having played with the numbers a lot myself, I'm not sure where you get 1.36 MH/$. If you're willing to run open air, and have all your hardware off one motherboard and buy PCI-E extenders, you could run (8-eight) on a single board, but then that decision implies other expenses, like 8 x $25 PCI extenders with power-taps to keep from cooking your motherboard, and framed cases, BIG power supplies, and motherboards that will support 8x PCI-E, all of which add to the bottom line, and offset the savings of the cheap cards.
I put the rough build cost on a machine like that at about $2500 by the time you include EVERYTHING you'd need, which is still around $1.07 $/per/mhash for 2400 Mhash.
In the example you give of a 1600 Mhash, which, at 300Mhash/per/5830, takes 5+ cards (depending on OC levels), which is still $650-$800 worth of cards, depending on your sources.
I'm not saying including the cases isn't hurting the sale, I'm saying that even $1/per/mhash is an aggressive goal.. 6990's have other advantages like Mhash-per-box, but $-per-mhash isn't one of them.
650-5x5830
10-1 gb ram
10-flash drive
75-5*extender cable
30-cpu
195-mobo(probably could go cheaper)
240-psu
60-open frame case made out of aluminum tubing
=$1,270
5 5830's set up in an open frame case can give you close to 330 Mhash
(5*330)/1270=1.29
That was a quick calculation it is in no way the most efficient you can go.
The problem I found with going higher then 5-6 cards is the parts in the server like the mobo and psu get exponentially more expensive so thats the sweet spot.