I don't think dual booting on your internal HDD is recomended at this point. You have to use vista and it's tools to resize the windows partion. google "dual boot vista" add the words sabayon, gentoo, and linux to each individual search this will give you some links to do it the right way. read as many as you can. Hosing a computer is easy. read and understand what you are doing before you do it.
As for the external HDD when you install Sabayon it should be able to boot using the BIOS boot selector(should be F10 or similar, right when the BIOS screen shows up after a reboot.)
If Sabayon is installed on external HDD it should boot just like any other boot medium like your CD/DVD ROM, a USB thumb drive or your internal HDD
On the instalation to your external hard drive you should be able to run the installer and when you get to the partition editor assuming both your internal and external HDDs show up, uncheck every disk that shows up except the external HDD. when you install make sure you edit your swap to be twice the size of your ram. if 2GB of ram swap should be 4GB. search the forum and the wiki for partioning instructions.
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