I'm writing this on my personally built Windows 7 box. I want to tack Arch Linux onto it but I do not want to touch my existing hard drive. I have an old laptop hard drive I could theoretically convert and there's also an old desktop hard drive I could take out and wipe as well, assuming nobody in the house has any use for that computer anymore. Regardless, what steps will I have to take to dual boot with two hard drives? Will my bootloader load and give me a choice between the two operating systems, or will I have to modify the boot sequence in my BIOS to ask for a choice of boot device selection upon booting?

If I go through with this plan, I'm probably going to remove my Windows 7 hard drive from my case before I commence the installation in the name of disaster prevention and then hook it back up when Linux is loaded in. I would have Windows 7 hooked up to SATA 0 as it is now and Linux hooked up to SATA 1 once I complete the Linux install and put the two hard drives back into the case.

Assuming you'll be using Grub or Grub2, you can find the relevant info in the following thread:
http://ubuntuforums.org/showthread.php?t=179902

also here
https://wiki.archlinux.org/index.php...le_Hard_Drives

I use the more modern computers that have a choice to allow the drive to be moved in the boot order. I assume yours does.

That way I make a complete install to a selected drive. I take the second drive and move it to the first boot order in bios. Don't just pick it at boot time just yet with a F key. Move it in bios boot order under hard drives.

Then when you (carefully) install your linux to the now first boot order all the changes should happen to the what was second drive. You can not continue to boot in that order or change back either in bios or with F key at boot time option.

If you want the most safe way then consider using a virtual machine.

You should have a complete backup of your Windows 7 right now before you begin. Use a usb or network share to create it using the windows backup.

I did it the ultra safe way, which was to completely disconnect the Windows drive during installation of Linux. Once installation was complete, I reconnected the Win drive, selected the Linux drive as the boot drive in BIOS, booted and ran sudo update-grub, and everything just works from then on.