The short answer to the question of whether or not the Q9450 is a bottleneck to the system is, yes. But there's nothing inherently bad about that; every system has a bottleneck and if you remove the Q9450 bottleneck then it will just be some other component that's the bottleneck, right?
If the question is whether or not the Q9450 is a bottleneck to the HD 6950 in gaming scenarios, well, it probably is a little bit of one, but at HD resolutions and higher image quality settings it's probably not very much of a bottleneck. Of course a faster CPU would be faster, but I don't think a Q9450 is inadequate for gaming yet.
As for CAD, that's largely CPU / RAM dependent and doesn't tax the video card much. There are scenarios where it does and there are specific CAD programs that need a more nuanced discussion regarding the video card (Solidworks, maybe some others), but generally a low powered video card works just as well as a high powered video card. A HD 5670 1GB won't feel any different than a HD 6950, typically. In terms of RAM, 8 GB should be enough for the vast majority of situations. Even here at work we've got a couple people going on less but the most demanding users seem to get by fine with 8 GB (although with RAM prices where they are I'm putting 16 GB in new machines).
As for rendering, the most important thing is to have as many cores as possible. You've got a quad core so that's good. You can make a rough guess about how your CPU compares in rendering from right here: http://techreport.com/articles.x/21987/13
and my guess is that a i5 2500 is about 75% faster than a Q9450.
You're Q9450 is still a very capable processor and it's not going to hurt you to sit on it for a little while longer - unless you're pumping out CAD renderings for money. If it's overclocked then you're in even better shape. A Q9450 should be very close in performance to my X4 955 and I plan on sitting on that for a while longer, for what it's worth.
Lastly, it's never a mistake to purchase an i5 2500 K, unless it's really close to the 3670 K launch and you can manage to wait.