Regarding mining, I don't think people are very cost-efficient in how they setup their mining rigs. There's too much "I want to get the best there is" mentality floating around which increases CAPEX cost significantly (for less OPEX of course - but IMO it's not worth it unless the electric costs are too high). The fact that people are buying brand new 280s/290s and pay thousands of dollars from the get go is a bit disturbing.
Clocked 5850s and 6870s are pulling like 1.3 MHs* and people can find used ones at around 100$ per card instead of 500-600$ for a brand new 290. Will a 290 give 5-6x performance? Nope.
* For some reason, series 58xx and 68xx seem to have a factor of 3.6-3.8x compared to Scrypt.
Clocked 5850s and 6870s are pulling like 1.3 MHs* and people can find used ones at around 100$ per card instead of 500-600$ for a brand new 290. Will a 290 give 5-6x performance? Nope.
* For some reason, series 58xx and 68xx seem to have a factor of 3.6-3.8x compared to Scrypt.
Not sure how you're getting 1.3mhs on a 5850, mine start being unstable and risk crashing the comp when trying to push over 1mhs, but if you want to ignore electric costs and look at just hardware costs, you also need to inclue motherboards, ram, etc, since you'll need twice as many pcie slots to get the same hash power.
Regardless, electric costs are not negligible when you run them 24/7 for moths at a time, and although I agree with you about the 280x's and 290s not being worth the premium, I wouldn't buy any 5xxx or 6xxx cards at all at this point, as there is a significant efficiency step up to gcn, so 7xxx is the minimum.
