Upgrade CPU is generally only worth it if you are upgrading the rest of your rig, or if your CPU is very low tiered, which 860x doesn't seem to be that bad of a CPU. If you are only upgrading part of your rig, I highly recommend upgrading your GPU instead.
Also, note I mentioned upgrading, not Xfiring. I personally feel Xfire/SLI is only worth it for top few cards from each side, otherwise it is often better to simply swap the card for a higher end one. GPU doesn't scale that well and often you may have to deal with issues that could arise from such an arrangement. I have an SLI setup, and while I haven't had any issues with it so far, I would have prefered a single GPU solution if there was one available for me, which there isn't.