Cygnus x-1 said:
It's not bottle necking for the cad work, but it is holding the gpu back a little. Try overclocking it to around 3.0 to 3.2ghz, that should be a pretty good match for the two. you'll also notice a boost in cad work with the clocks being higher.
I have no experience overclocking, but I might as well try; would picking up a higher-clocked dual-core, like an E8400, have the same effect? I had originally picked up the Q6600 because I was doing a lot of freelance rendering work, but that's no longer the case.