I can get the 6870 for $180, but at the same store, for a hundred dollars less, I can get a second 5770 which will give me better performance than a single 6870. Is there any reason(s) why crossfiring is a bad idea?
I am running crossfire hd-4870's, A6-3650 firstname.lastname@example.org/550mhz app shader.
It seems to play games well, mostly you have to enable "v-sync" so it runs smooth.
1080p gaming is decent. games like dirt2 run at 43min, 67avg with highest details, 4x af, v-sync off for testing.
If you are running a dual core cpu moving on to crossfire wont help much unless the game supports app opencl then it might use some shaders to help the cpu out.
If you have a tri-core or quad, yes crossfire does help.
My board is a micro atx x16-x4 board. The old x2-4400+ 939 crossfire board was full dual x16 speed slots. I did a few benchmarks. In the ones that test only the gpu power the old board beat the new one by about 5%.
I suspect using larger cards in crossfire with a x4 second slot would make this bottleneck start to matter more. So maybe a pair of hd-6870's might microstudder or something.
In catalyst (vision in win7) you simply check the enable crossfire box and crossfire is good to go.
Make sure you keep the gpu/mem clocks the same on both cards when you overclock them in overdrive. Yes you can even overclock each card when running crossfire.