Install and boot Windows from USB hard drive?
As far as I know Microsoft lock their Operating Systems so that you cannot run them directly from an external storage device. Microsoft say something about it not being a 'primary means of storage.' More like them making an attempt at stopping people running multiple computers of on copy of the OS, as allowing them to install it on any storage would allow people to make loads of copies. Hence, Linux does not have this problem. I personally do not do much gaming, and have never needed Windows on my MacBook Pro, so have not dabbled in the whole 'Windows on my Mac' thing. If you really do have a problem with sticking Windows on the MBP's Hard drive and booting either natively or through virtualisation, then the following links may help, although the guides are designed for XP and I have no idea if they will work on 7.

Prepping the install CD (scroll down)

Installing

As I said, I cannot guarantee this will work for 7. I hope, however that I have helped in some way.