Using External Hardisks as Boot

I got something to ask. It's about how is it possible to use external hardisk as a boot, running other operating system instead of OSX. The case is, I have one macbook pro with 500GB space of hardisk. and I use 250 for installing all the production softwares which is space-taking.. and the rest for scratch disk and for smaller use-able data.. but it's running just for snow leopard, I want to use be it with a bootcamp or virtual machine other OSs such as windows 7 because I want to use 3DMax, while trying and testing ubuntu/fedora (unix based OS).. So how should I configure the external hardisk for above purposes.. So I mean, running other OS (optional/particularly) stored on external disks.. If anyone has a tutorial/walkthrough/step-bystep.. please kindly let me know then... before!