the cards will run independently form each other at their own respective speeds. The catalyst drivers stopped 'match-making' cards clock speeds long long loooooooooooong ago.
and yes only same-series cards can be x-fired efficiently (afaik you cannot crossfire cross-series)
my rig, though over a year old already, will last another couple of years
unfortunatley :roll:
no need to replace anything unless it breaks before the 2013
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tbh my next upgrade/change will probably be the OS
well your settings look ok for the memory, what about cpu voltage, you say its overclocked and again if the IMC isn't brilliant, memory may fail, you may need a touch more voltage - what are your current setting for cpu?
qpi/dram voltage is related to the CPU IMC also, overclocking the cpu may well make the memory unstable unless you up the qpi/dram voltage also.
what is your uncore speed set to?