Hmm, hard to say, that CPU packs some serious punch, even if it is a bit old. It's not as though you're talking about putting a 4850 with a Pentium 4 or something like that.

That said, I've heard rumors that ETW really does take advantage of multi-threading, and the E6400 is a standard LGA 775 chip, so popping in a newer proc should be easy enough. I'm considering replacing the proc myself, just to see if it can make those interminable AI turns go by a little faster ....

But on the face of it, no, that CPU should not bottleneck the GPU. At least I don't think so.