Will a GTX 260 bottleneck a C2D E8400

Not really. The GPU won't bottleneck the CPU, or vice versa. The e8400 runs at 3 GHz, which is pretty much the clockspeed a high end GPU needs to run at nearly its full potential.

A e8400 will not bottleneck a 260 or a 280 to any noticeable degree.
 
A fast CPU like the E8400 will not bottleneck a Single GPU anytime soon. It's when you go multi-GPU that the CPU will start falling behind.

If you only have a single card (the wisest decision in my opinion), the E8400 will not bottleneck any GPU.

Plus, let's say that in 2 years or so, the E8400 will start bottlenecking the fastest GPUs around at that time; you just bump that beast to 4ghz and there u go, no bottlenecks again for a long time.

The E8400 and all the other high-end Duals will last a good while for games, I believe. They're pretty powerfull little beasts.