I guess people without or very low power concerns can use the 5830 method and those with the high power usage would look more towards the 6990s. You can build a complete rig with 4 5830s for 800ish bucks and get more than one 6990 mhash. the downside is your are "wasting" power for other parts (mobo, ram, cpu...). so if you add 4 6990s at 3000 bucks you can build 3 nearly 4 rigs for that. lets say three rigs at 1200 mhash a pop totals 3600 mhash. 4 6990s on avg will pump out 3000 mhash (keep in mind you may need to spend for an additional psu changing the equation a little). So if power is primary concern then the 6990 might be the way to go, but for myself and others power is not a concern 5830s solution is the way to go.
The problem is you have to wait a very long time for the 6990 to make up the price difference between the two cards. Lets use the 5850 for example since that was the last 5xxx series card to have a great deal.
I think its fair to say a 6990 can generate 900 megahash with a healthy overclock. A 5850 can generate 390 with a healthy overclock. That means you need 2.3 5850's to get the same megahash.
A 6990 requires around 330 watts at full load. A 5850 requires around 170. 170*2.3= 391 watts for the same ammount of megahash.
This next part will be different for every one. If I use my 6.7c a kwh it would take much longer to recover the costs and it would be much faster if I used 25c a kwh. I'm going to use 15c a kwh because I remember hearing thats the average in the US.
So the difference is 61 watts which is 22 cents per day, 6.60 dollars per month.
2.3 5850's would cost you 145*2.3=333.50
The difference between a 6990 and 2.3 5850's is 401.49
This means it would take 1,825 days or
5 years to make up the price gap between these cards.
