Just go ahead and install Windows 2000 on your second hard drive, and it will automatically creat an entry in your Windows boot file (Boot.ini, in the root of your C drive). If you want to manually change that file, boot into Windows XP, right click on My Computer, go to Properties, click on the Advanced tab, and at the bottom under Startup and Recovery, click Settings. Then, under System Startup, click Edit.
If you want to use another bootloader besides Microsoft's one, just do a search for operating system bootloader, or something along the lines of that.
Yup. Just install 2k on your second HDD. Everything else is automatic. Easy as pie. Another option (what I do) is just run virtual PC, and load whatever you want into that 'system'.
Honestly though, if you have WinXP Pro, why would you want 2k? If its the GUI, there are options to have XP Pro look the same as 2k, and MOST important, from my understanding of the issue, 2k DOESN'T support HT. I'm my experience, dual bootin is a waste of time. (Unless you really, really, really need it...)