Any internal upgrades short of another 12gb of RAM would be a waste of money, as Jack said. That's the blunt truth until technology advances again in the future. There's no way that the i7-980x will give you anything close to another $1,000 of performance.
Maybe you'd want to try liquid-cooling or something like that... takes a lot of time and energy to do, though, and isn't nearly as maintenance-free as a good air-cooler.
I'd say go for peripherals. If you don't already, get an Eyefinity setup of three 1080p monitors.
If you do, get a soundcard and a pair of the best headphones you can find. More specifically, the Asus Xonar Essence (although I don't know if your PCIe x1 port is still available with the three 5870s), and a pair of AKG K701s. You'll want that card for those considering the power they need, and the fact that it has an integrated headphone amp. You'll have to check and see if that x1 slot is still usable, though... If you're a speaker guy, go get a Bose Acoustimass system and the Creative SoundBlaster X-Fi Titanium Fatal1ty Professional card. Either one of those would really make everything ,more entertaining if you don't already have a high-end sound system.
If you already have all that, go out and buy the most expensive office chair you can find, because your PC is the absolute best it can possibly be.