Booting from USB drive (noob question)

Hello everyone. I'm trying to flash my ATI HD5770. I read up and did some research and I think I can pull it off. However I have a (probably) very stupid problem.

I have the HD 5770 plugged in on my Mac Pro 2008 as I type this so I can't log in OSX of course and I can't access the boot option by holding the command key. I have bootcamp booting my Windows partition automatically. My question is how do I get it to boot from the usb pen? I have the USB pen ready and it should be bootable but where can I set it to look for the pen before accessing the Windows partition?

Thank you!!

 

Thank you for your reply. I formatted the drive following a guide in a forum using the HP format tool and using bootfiles linked in the thread. As you might have noticed I'm no expert so what you explained next is close to chinese to me

Thanks again! I'll try to do more research in the meantime