I run it (AutoCAD 2000 on Windows 2000 on VPC 7) on my iBook G3/900/32/640. It is usable, although there are couple of minor bugs (one of which affects how the cursor appears). I generally don't use it for the majority of AutoCAD work as a I also have a PC desktop, but do use it if I am on the road and have to make changes to a drawing.
So you can definitely do it, and because AutoCAD is not the demanding processor wise you can probably do alright even if your machine is isn't that much faster than mine.
As previously posted, stay away from XP, it's too heavy, but if you have to run XP, it will work, just make sure that you turn down all the extra appearance cr$p that Microsoft worked it.