Adding a new drive would mean not having to do anything to the existing
one. And I agree, backup in any case. You just have to be comfortable
removing drives and setting the jumpers. The steps would be:

(shutdown, unplug)
1: Pull out existing HDD
2: Install new HDD
3: Insert windows install disk, power up, proceed with install
(shutdown)
4: Put linux drive in master position with jumper set to master
5: Put windows drive in slave position with jumper set to slave
5: Boot into linux and update grub

I think it's pretty tidy, and easy enough for a beginner. I'm sure you can
do this without removing the drives, but I'm not too familiar with the
windows install process.

As far as rewriting the boot loader, like I said, I wouldn't want to have to
figure that one out as a newbie under a deadline. This way he can swap the
drive positions and *know *he get back to work in minutes, and mess with the
boot loader later. Not that it's that difficult, but it's a big added
stress to have to reinstall a boot loader you know nothing about when you
have real work to do. And this kind of thing always happens at the wrong
time, for me anyway.

I also agree that setting up a separate /home partition for your linux
system is a good idea, this will be easy to do if you decide to reinstall
your linux system. Somewhat harder from an existing system, but there are
step by step tutorials in the forums.