HD 6870 is aroung 20% faster than the HD 6850. It's worthwhile to purchase the first. The only problem here, is that the 6870 needs two 6 pin conector from your PSU the other needs only one. I bought a 6850 3 weeks ago, and I was more or less in your position. The 6850 cost 99 euros and 6870 130. I chose the first because of the PSU problem and because the HD 6850 included Dirt 3 and Deus Ex. Nevertheless I have to say the HD 6850 is great. No regrets at least up to 1680x1050!
My 6850 card alone scored slightly higher (17630 3Dmarks vs 17383), but when I
put it in crossfire with the 8670, I scored 19192 vs their result of 23681 for 2X 6850 in crossfire.
I assume the 6870, installed in the PCIe #1 slot, will be seen as the master, while the 8650 in the #2 will be the slave. That's where I started, and hence, the
One of the (other forum) guys told me the second card should be in the PCIe #5 slot. Really? I mean, why would that make sense? Anyway, I tried it without
any measurable change.
Also my Win Dex graphics dropped from 7.8 for the 8670 alone to 7.7 when in crossfire, which was where I started when I had the 6850 running alone.
(strictly a reference point)
I'm also looking at FRAPS during game play, and get consistent 60fps, and then 100fps when I turn off vertical refresh rate in my first test, depending on the game.
I had a decrease in performance when benchmarking the crossed and uncrossed configurations within the DiRT 3 benchmark.
The 6850 did fine alone
The 8670 did better alone
The two cards in crossfire did not increase performance. They together preform at a lower fps than the 8670 alone.
And, in crossfire, they are not even close to results I've seen for 2X 8650 in crossfire, which is what I would've expected.