Well, 2000 and XP both had bootloaders that would load, and then you could choose the OS you wanted, and as long as the OS didn't have to be on a specific drive, the bootloader didn't care, nor did the hardware. So for example, you could have 2000 on your first drive, and XP on another drive, and still boot into XP. XP's bootloader would be on that drive that 2k is on though. Linux's bootloaders are much the same in that respect. That is why I wondered if something different happened with Vista.
As for choosing in the BIOS, you can do that with sata drives no prob. But better yet is pressing whatever key you see around POST that mentions Boot Menu or something similar, that way you don't have to physically change anything. There just is a menu that comes up, independent of the OS that you then choose what drive you want the system to boot from.