Your machine seems a real performer! How much time does Rentertest takes in 800x600? (I feel curious
Also, I am not sure if TurboCAD handles this as some other 3D modelers, but high-end professional video cards provides anti-aliasing for Wireframe and hidden line views. This is a functionality not available with common gaming graphics adapters. And if you like to have a preview render in OpenGL, pro graphics are known not to stutter when you rotate an object or you move in a scene.
These are the only advantages of a pro (lots of $$) versus a gaming (about 250$ for a recent one).
The advantages of a fast gaming card is only noticeable in OpenGL render mode.
Also, I strongly recommend (as AMD/ATI are the same company / affiliates) That if one buys an Intel i920 to i940 (neat processors) that he shall combine it with a nVidia video adapter. If one buys an AMD phenom, get it combined with an ATI. Less "oh that thing doesn't work well because of the combination". This may avoid any driver problems. I'm not saying that you are running after trouble if you do otherwise.