I was running AutoCAD 2000 on my old Pentium 133 laptop with 24mb RAM and it ran it just great. Didn't multitask well, especially when working with multi-megabyte drawings, but it did impress me how little resources AutoCAD actually uses!
At any rate, I upped my 3.2GHz PC (windows XP) from 512mb to 1GB because I do a lot of 3D CAD and I didn't notice a lick of difference. What is extremely nice, though, is having dual monitors- main window is on my 21" CRT running 1200x1600, and my 19" LCD @ 1280x1024 has 2 or 3 3D viewports on it. Oh man it's great when you're doing things in 2D, I'd highly recommend it! For 2D, it's overkill though.