Windows 7 when installed last to another drive should see XP and correctly configure a Dual Boot Menu with the boot-critical MBR files updated on XP which remains System Active.
Boot the Windows 7 DVD, select Custom install, use Drive Options to format the Windows 7 drive before install. The DVD will scan the drives at bootup and see XP to allow Upgrade key.
However this makes the drives interdependent and harder to extricate when ready to get rid of XP. We help do this all the time here, so you can find help when the time comes.
If you'd like to make the HD"s independent so they can come and go as you please, then unplug the XP drive to install Windows 7, replug, set Windows 7 as first HD to boot in BIOS boot order, then access XP by using the BIOS Boot Menu shortcut key given on first bootup screen.
You'll need to use this workaround if you do that since XP won't be seen by the Windows 7 installer: http://community.winsupersite.com/bl...he-answer.aspx