1. It depends on your PC's BIOS. Get into BIOS and look at the options in the boot sequence menu. If you see USB listed as an option, then you can just move it to the top of the list, save your changes, and then anytime the USB drive is attached, the PC will attempt to boot from it first. If your BIOS does not support booting from USB, then you are out of luck.
2. You shouldn't need to change anything.
3. One that is compatible with both your existing PC and your budget. There's no real answer to this question.
4. Yes. I had installed Windows XP and SuSE on my laptop, but when I upgraded the laptop's hard drive, I put the original hard drive into a USB enclosure. When booting up from the USB device, I got the expected lilo menu. If you do this I'd recommend installing the boot manager to the master boot record (MBR) and installing Windows first, then Linux.
5. Assuming you install your boot manager to the MBR, you'd have one boot loader on your internal drive and one on your external. To use an analogy, suppose you have installed Linux on your machine, but then try to boot from a Live CD such as Knoppix. Assuming you've set up your BIOS to boot from the CD before the hard drive, then your system will boot from the CD. Anything that's on the hard drive is irrelevant (at least until booting finishes). In other words, if you are booting from device A, the boot process does not first attempt to read from device B, then go back to booting from A, and maybe making some sort of conditional decisions downstream. As long as a given device is bootable, and you set up your BIOS to boot from it, then whether or not there are other bootable devices also attached to the PC doesn't really matter.
The key to all of these questions really comes down to whether or not your BIOS supports booting from a USB device. If it does, you should be golden. Good luck with it.
Edit: note also that in order for your proposal to work, your system architecture must be compatible with Linux and Windows, with both are x86. If your Macbook is using an Intel chip, you should be OK
Last edited by J.W.; 11-22-2006 at 04:18 PM.