I've used all versions of autocad, the windows one does seem to be more responsive and use less resources but it has been around for years. The mac version is sometimes slow - mostly just when loading up and when no document is open as it fades out the tool palettes.
Overall it's such a good program to eventually have on the mac and means no more windows partition for me! YES!
However, some features are a little too simplified, but it does boast a few more cool features and easy to navigate then the windows version.
running on a 4gb ram 13" uni macbook, 2.4ghz, 500gb HD. paired with a 22" monitor dual desktop. - Works well

The latest drawing I'm working on took up 2.5gb ram when open - however this had like 421,000 lines in though so fair enough! (some drawn, some exported from google sketchup pro 8)
I would get it if I were you, the updates that will start coming soon (I guess) will only improve the program.
Luke.