You did NOT take the total system cost into account.
 Try adding $40-$50 to the price of each card THEN compare - and that's being optimistic as I don't think you can build a viable *8 slot* system for $320. It's hard enough building a viable *6 slot* system for $320, if at all possible.
 
 Dual mine of DCR does in fact impact ETH hashrate - that's been WIDELY reported on the Claymore dual mining thread - though it's not a big impact, 10-20% per most of the reports.


 I do agree that relative profitability can vary - ETH in the last week has kicked up a lot more than ZEC has as a percentage, which is giving a bigger boost to ETH mining than to ZEC mining which is at least for the short term boosting the profits on AMD cards more than NVidia cards, at least 'till folks start building enough rigs to drive the diff up to match the price increase.


 It would have been interesting to see how the numbers compared a month ago, when both ETH and ZEC were a lot lower priced and fairly CLOSE on price (and both had a bit less network hashrate).