Not only is is possible. But the cards in question are designed for just such a setup. A normal crossfire setup will work just fine as long as both pci-e lanes are x16. It will work with an x8 setup but ideally you should look for a mobo with at least two x16 pci-e lanes to get optimal performance.
I currently have an Asus P5K P35 chipset though ( ), which I believe does have two PCI-E x16 slots - however, one runs at x16 mode and the other at x4 mode. I've heard bad things about the P35's crossfiring capabilities.
Would a single gpu 3870 and a 3870 X2 run alright on a P35 chipset?
Yes you can crossfire them. You need to consider though that your board has one 4x slot which will slow down communication between the 3870 and the 3870X2. Also I believe the bridge between the two 3870s on the 3870X2 is a PCI-E 1.1 switch instead of a PCI-E 2.0 switch that's on the newer X2s. The result will be a 4x PCI-E 1.1 link between the 3870 and 3870X2 giving you alot less of a performance boost than you would get if you had two full 16x slots, or even two 8x slots. I suppose with 3870s falling bellow $100 now it would make an interesting experiment.
Thanks for your replies phin and mexamanx00; they've really made everything about crossfire much clearer to me!
I am a bit worried now about the P35 chipset. But now that I know I can crossfire with three gpus, I'm glad I won't have to discard my single 3870 if I wanted to progress my way slowly to a dual X2 setup.
btw, do you think a three-way setup with the 3870 and 3870 X2, even on a 4x PCI-E 1.1 link, would out-perform a single gpu 4870 game-wise? According to lostcircuit.com's review, the 4870 out-performs the 3870 X2 in games (though the 3870 X2 wins in 3dmark).
But thanks a bunch for answering my first questions; just trying to further gauge the anticipated performance of this potential config
Your performance with a tri-crossfire setup would put you ahead of the 4870 in most cases. Tri-crossfire is the best config for ATI as of now due to Vistas threading limitations. Three 3870's are more powerful than two 3870's AND more powerful than four 3870's due to limitations in the operating system and how the drivers react to its environment. AMD is working on a fix for this. They also have windowed mode crossfire on their road map. We will just have to wait and see what the future brings in driver optimization.