AutoCad is available now for Intel Macs:
Previously, version 12 ran on older Mac OS versions. If you find it doesn't work well enough for you in Classic (the Mac OS 9 environment you can run within Mac OS X on 2005 and older Macs), and the newest version is too powerful for your current Mac, you may want to consider one of these alternatives which read and often also are able to write AutoCad files. These links to their webpages are given here so you can further research which product best suits your needs:
The website http://www.architosh.com/ is a great resource for CAD programs and users of CAD on the Macintosh platform where you can check for reviews and user opinions about the various products.
I don't use any of these products, but have been keeping track of available software in the field since I do mapping which uses Computer Aided Design programs, and have kept an interest in seeing what is available for the Mac in the field.