Removing the Xp HD during Windows 7 install gives you the cleanest Dual Boot configuration, since it boots via the BIOS and doesn't intertwine the HD's as a Windows-managed Dual Boot (created by Windows 7 when both HD's are plugged in) does.
After Windows 7 install, plug XP back in and set your preferred HD to boot first in BIOS, after DVD drive. Then use the BIOS shortcut key for Boot Menu given on the first bootup screen (often F8, F10 or F12) to boot the other HD instead.
Neither method will hurt XP, although if it remains plugged in during Windows 7 install, the MBR on XP will be updated to manage the Dual Boot with Windows 7, making it harder to extricate XP later without special steps which we can help you with.