New GPU = performance benefit..?

[Shonumi]

Have a look at the Minimum GPU Specs thread NaturalViolence made. It'll help you understand how well a GPU will do in Dolphin. Note that in Dolphin, the GPU is responsible for things like IR, AA, and AF. The rest of the performance, for a lot of games, comes from the CPU. I think the ATI 5870 should be enough for 4x IR (according to the thread I mentioned). I assume the GTX 680 should be able to do that too, though I'm no expert on GPUs. It won't help you run games faster, but it may allow you to up Dolphin's graphical enhancements. If you're good on the 5870, I don't think it's worth upgrading in regards to Dolphin emulation.