I design in 3d mostly, but occasionally have to draw up in AutoCAD, since I'm a ME and not a CE I don't use ACAD much therefore I HATE it (Yes I know some MEs still design in ACAD and that's largely due to gigantic file systems all in 2D that would cost too much to convert to 3D). I don't have much to compare with other than comparing my workstation to my home PC for 3d modelling (SolidEdge)
Workstation - Intel P4 w/ HT 2.8Ghz & ATI Open GL GPU (Forget which one) & XP 32-bit
Home - Q6600 @3.2Ghz & 8800GT, XP 64-bit
Anyway my larger 3d assemblies render ridiculously faster at home despite having a gaming vid card. Of course this is largely due to a far more powerful CPU but it appears to take advantage of the cores quite well.
I realize that this doesn't help at all. But until AMD comes out with a Phenom that works at clock speeds as high as Intel, I wouldn't even consider an AMD CPU for CAD.