are you overclocking by increasing the voltage? gotta becareful..
i have mined with my 6870s for more than 2 years and none have any issues, just resold all recently at 60%+ cost basis, actually 75% if you count the included free game coupon that came with the card that i sold back then. And have upgraded to r9 270-280
You need good cooling to make sure the chip is at low temp and fan isnt going at full speed all the time, mine is kept <70C at full load and about70% fan rpm. Also dont buy the most highend high power video cards those are way expensive, loses more value on resale, and generate tons of heat. Get the mid tier version which run much cooler, the r9 270 is my favorite it runs 100% stable and cool.
Also get one of those, run it on max and blow it directly at the intake of the video cards, it does wonders.
http://www.amazon.com/Stanley-65570...466328&sr=8-1-fkmr0&keywords=stanley+blow+fan