Yes, you can dual boot from different hard drives.
Yes, it is semi-difficult to get Vista to install to a different drive if you don't know what you're doing.
Vista will complain (loudly) on an older hard drive, and it'll make your anti-virus programs take much longer to complete.
You can even dual boot between X32 and X64 OSes.
A physical switch will keep random data transfer from occurring, but it is handy to keep both OS's files accessable from one or the other.
For the record, I dual boot Win-XP x32 Home SP2 and Win-Vista-Ult x64 on my system, from two separate partitions on a 250 GB SATA. So I can't see why two different drives wouldn't work.
EDIT: If you install a "switch", no. It physically turns off the power to one hard drive or the other depending on how the switch is set up.