I'm really on the fence with this the more I think about it now....
There may not be much in it between the GPU and CPU as to which what over shadows what, it's probably a balancing act at 1080 which makes it such a tough choice. Given the reasonable second hand prices of other processors which can drop into your mobo it might be tempting to do this. Equally I think it may be worth seeing how much you can get for the 4890 and offsetting that against a newer graphics card. As it's a P5 Mobo perhaps invest in the graphics card now in the acceptance/knowledge that at a later date the mobo/CPU will need upgrading in any event so at least you have one part of the rig that is current.... assuming you intend to continue upgrading your rig rather than buy a new PC later down the road
You know how to create a dilema don't you?