The biggest problem with Parallels on the Mac is that it hasn’t been able to properly take advantage of the graphics capability found on the Mac — so even if you had a Mac that had discreet graphics with some kick to them, Parallels really couldn’t do anything with them. That will almost certainly change in the future as Parallels continues to improve, however.

If you want to run AutoCAD on the iMac hardware, your best bet would be to run it under Boot Camp — that way your system is effectively running as a Windows machine, with no pesky Mac OS getting in the way and taking up system resources. And you can even run your Boot Camp partition through Parallels using the newest versions of Parallels, to avoid having to double-dip into your hard drive to run Windows under both modes.