2.0 and 3.0 defines the gen of the pcie. on a mobo all slots would be 2.0 or 3.0 . what you really wont to ask is if you can crossfire gpu on different pcie speeds!(for example pcie x16 and x8)the answer is yes. running crossfire in one x16 and another in x4 isnt recommended, performance will be cut down severely! pcie is backwards compatible but if it can operate as 3.0 why would anyone want to run it at 2.0, except due to gpu limitation.