Have you even used Parallels? There is very LITTLE overhead from OS X unless your running many other programs inside of it. When running both operating systems at once your VM sees a 30% performance hit. (I've heard of 20% running using a Linux host.) I run XP just fine with 128-512 MB of RAM.
Yes I have, and whether it performs well or not doesn't change the fact that Windows must be booted, and thus you are running both Windows and its programs, AND Mac OS X and its programs. This is obviously more overhead than a situation where 1. You aren't running OS X or 2. Don't have to run Windows itself in addition to its programs.