Which pci slots can i use for crossfire?

You can't use any pci slots for a Crossfire/SLI set up. You can however, use pci-e (PCI Express) slots for your X-Fire/SLI gpu's.

As far as what the best slots to use are, read the mobo manual. If you do not have the mobo manual, you can download it from the Asus website.

Also, please specify which Asus Rampage Extreme you have; is it the Rampage Extreme or the Rampage Extreme II?

If you have the Rampage Extreme, your question is pretty silly considering this mobo only has two pci-e slots to begin with. But, I presume you have the Rampage Extreme II because that actually does have 3 pci-e slots. If in fact you do have the Rampage Extreme II, then typically speaking, you want to set up any X-Fire or SLI configs using the same colored slots.

Lastly, it helps to fully list any and all system and hardware specs when asking questions; especially if you want an answer specific to your situation.