There is always a bottleneck somewhere. Right not its your video card.
E6600 is still a decent CPU. And with the ASUS P5W DH Deluxe you can probably overclock the CPU some.
Will it be a bottleneck? Possibly. Will your gaming performance increase substantially? Yes.
You could get the 5850 (if you can find one) and enjoy an improvement in your gaming experience till you're ready to replace the rest of your system. The 5850 will migrate nicely into the new system.