Why is that the only option? Why not trade the q6600 for an e8600 (with cash difference) and o/c it? You will hardly see a bottleneck then, even with a 280. Rather get a high end dual core than a low end quad for gaming purposes right now. i7 is incredibly powerful but right now it's not so much of a difference in gaming unless you can afford to sli 280's because the bottleneck is mainly on the gpu.
The op will still have more or less the same sli problems his always been having. A single card solution isn't really better.
Rather get something like the gtx285. If you're gonna be keeping it for a long time rather get a 4870 which at least has dx10.1. Then you can get a DX11 card at the end of the year.