As far as a mobo processor I think every one would recommend a 2500k/z68 motherboard. This would also mean that you would have to upgrade to ddr3, which is ok because ram is dirt cheap right now.
As far as video cards, in order to get a significant upgrade over the 6870 you want to go at least 2-3 tiers up, so a 6970/gtx 570 at the very least.
That's if you want to stay with the single card solution. If you decide to go with the processor/mobo upgrade you could just pick up another 6870 and crossfire them (which is the bare minimum I would crossfire with in order to avoid microstuttering