As far as i can tell most of the time there are around a 15-20% difference in performance between the two. if you can justify paying the extra cash just for the 5870 name alone that's cool. atm. you'r loosing around 10-15% "performance per buck" for the 5870 vs. the 5850. myself im getting the 5850 since i'll be using it just as a temporary card till the new models arive and drop in price.
If it were me I would wait. The new 6xxx series is right around the corner. Then you could get a 5850 or 5870 at a great price shortly after. If you can't wait , I would get a 5850 and oc it to 5870 speeds. My reference 5850 does 850/1250 on stock voltages. Of course your mileage will vary on the oc.
5850 is best between the two in price/performance ratio. But i would wait for 6XXX then buy a 5850/5870. The higher cost for no performance gain is going to be bad. Projected cost of 6770(performs on par/worse then 5850) will be higher then a 5850
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