Hello. Just wondering if you can not only mix gpu manufacturers when crossfiring, but if you can also mix gpu models, eg 4870+4850??
You can crossfire
any model within the same family and first performance-rating number (if the first two numbers are the same, you're good to go).
This means 3850+3870, 4870+4850, 4650+4670, 4770+4750, and so on.
The Crossfire system will only use the amount of memory available in the card with the lowest amount of memory available.
If you do HD4850 512MB + HD4890 1GB, the graphics subsystem will only use 512MB. If you do HD3870 512MB + HD3850 256MB, the graphics subsystem will use 256MB.
This happens because of the way Crossfire works. Most games run in Crossfire through AFR, or Alternate Frame Rendering.
It consists in having both graphics cards rendering the exact same scene and alternating the ouptut of each graphics card.
In a HD3870 512MB + HD3850 256MB setup, if you have a HD3870 512MB rendering frame 1, frame 2 will be rendered by HD3850 256MB.
If the scene rendered by the HD3870 was occupying the 512MB, the same scene wouldn't fit in the HD3850's 256MB, so some textures would be missing. And because everything happens too fast, there's no time to flush textures (or polygon or shader data, whatever) from the main memory to the HD3850.
So in order to maintain balance, the HD3870 512MB will only use up to 256MB, if being crossfired with a HD3850 256MB.
These are the rules.
There are some exceptions, though. Since they're identical, you can also mix the HD2600s with the HD3650 or the HD3850/70 with the HD2900XT/Pro. But the shader compatibility will be scaled back to DX10 (again the lowest factor).