AutoCADder

If you happen to choose a new PC for your AutoCAD, Inventor or other CAD application, grab that with the new dualcore technology. Dual-cores are good both for coming Microsoft Vista and for 2D and 3D CAD apps. Even a lower clocked dual-core CPU brings more power and faster CAD operations than an old single-core CPU with higher clock frequency. And this applies also for applications which are not multithreaded (like e.g. Inventor) - i.e. which do not directly make use of the multiple CPU cores. Operating system always performs tens of different tasks in parallel - and if it can dispatch those tasks to multiple processors, it helps also for the main running application.
For AutoCAD - and AutoCAD based applications - it helps even more as parts of AutoCAD are multithreaded since version 2000i. There is a good overview of what operations in AutoCAD can use multithreading - see Using multiprocessing in AutoCAD.