when you install windows on your new HD you can set up a custom partition scheme. For example if you have a 250 gig drive, you could do 50 Gigs for OS and applications and 200 gigs for data. You can even map your my documents folder to the secondary drive.
For the master/slave thing, they are jumpered (only applies to IDE drives) and are usually labled. There should be some sort of documentation with your new HD
As for installing Unbuntu. Download it and run through the installer, and it will automatically detect that windows is installed. You will then configure your boot loader during the installation wizard. Then when you boot your computer up you will have the option to boot into linux of winxp
it is a lot easier than you think, also check out the unbuntu forums, they are a great resource and there is a good community of people over there that can help you with specific unbuntu questions. I installed unbuntu on my laptop but ultimately went back to Suse when 10.1 came out. Its just a preference, so i am familiar with it, but I don't use it on a regular basis.
