New HDD/OS question (Ok, several questions)

Hey everybody - I just was wondering if anyone could help me with a question I have. I'm going to get a 2nd HDD for my PC, and I'm also thinking about upgrading to Vista soon. First, is it possible/feasible to have XP remaining on my current HDD, and use the new one for only Vista? If so, are there any problems I'd be likely to encounter?

Secondly, is this even an option I should be considering? I mean, I don't know if adding a 2nd hard drive is even smart in the first place. I currently have a 200gb 7200rpm drive that is pretty near full capacity, which is why I'm looking for another. Would it be smarter to just buy a 500gb drive and have only one OS?

Along the way, I guess I just never learned much about hard drives and the options we have with them, so I thought I'd come here to see if anyone could help. 

1. Dual-booting on seperate HDDs is feasible

2. Having a 2nd HDD is actually a smart move

Typically, most sensible professional users out there would prefer to run both OS' from the same HDD and keep the other as backup or a junk collector.

There are absolutely no issues or compability problems having 2 (or more) operating systems on one hard drive, or multiple hard drives each housing one OS on one computer, or any number (like 3 OS on one hard drive, 2 on another, with 1 on an external USB drive).

It is generally recommended that each OS be on a different physical drive because it just makes things easier when the time comes around to reinstalling and reformating the drive. You wont have to deal with multiple partitions on a single physical disk. Its not that its difficult to deal with multiple partitions, its just... You don't have to deal with it at all if they are on different drives.

Personally, if your system is relatively up to date (like, a Core 2 Duo/Quad, and a 8X00 series nVidia card), you probably shouldn't bother keeping XP around.