I can almost guarantee that if you are an average consumer, you won't even notice the difference in performance between the 860 and the 880. Wht you have really is a fast, awesome chip that you should be happy with. If you're not the average consumer and you feel that you really need the increase for whatever it is that you do, the 870/875 will be the best value that you can get. But keep in mind that if you pay $250 for a processor and then $300 for an upgrade, you just paid $550 for $300 worth of performance in your machine unless you plan on selling or recycling the old processor and putting it into a new machine.
Intel Core i7-875K, Asus P7P55D-E Pro, Win 7 Home Premium
MSI GTX 560 Ti OC, Mushkin 2x2GB DDR3-1333, Corsair TX650
Cooler Master Hyper 212+, Logitech Z-2300, ASUS Xonar DX
Samsung Spinpoint F3 1TB, Dell Ultrasharp U2410, Antec P183