using bootcamp with windows on external hd?

Is that possible? Can I run windows on an external harddrive with bootcamp? Would bootcamp need to be installed on my external or my internal? And what format should the external be so that I can run windows on it? And lastly, is 23 gb enough of room to run windows on the external?

I am fairly certain that it has to be on the internal drive

Found the answer in the Apple Boot Camp frequently asked questions.

http://docs.info.apple.com/article.html?artnum=303572#faq0

I have more than one hard drive, can I install Windows XP on any drive?
With Boot Camp Beta 1.1 or later, yes, but you can install only one Windows partition on any of your computer volumes. The Setup Assistant will allow you to partition drives, or to install Windows XP on an entire drive if it does not contain Mac OS X. Note: You cannot install Windows XP on an external hard drive.