How to dual boot with Vista x64 installed first

I would like to dual boot with Ubuntu x64, but I do not know how. I have never dual booted before. I read up on how to dual boot with Vista and Xp, but that was with xp installed first which I do not have or want. I am currently running Vista Ultimate x64 on my HP dv6000 Laptop, with 2gb of ram. Also can I do a dual boot with Ubuntu x64 and have it installed on an external hard drive instead of my C drive? Thanks in advance. I have learned a ton from this forum already and I appreciate everyones desire to help.
 

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