There's also a few other things to mention about an XP/7 dual boot with XP on first. Once 7 takes over as the default OS when the installation is finished the XP primary will be looked at as the boot/system when booted in 7.
The XP primary will need to be mounted in 7's Disk Management tool and assigned a drive letter if both drives are plugged in when 7 is installed. The EasyBCD tool will easily remove the "previous versions of Windows" item from the 7 boot options menu where as the tutorial points out you can then add the new entry in for XP.
Make sure to select a comfortable drive letter there from the dropdown list like D, E, or F. D is often the recommend. The alternate method for seeing XP added into the 7 BCD store when the XP is unplugged during the installation is a little more involved where once plugged back in both installations will be separate.
You then need to uncheck "hide the protected operating system files" and "files with known extensions" items in XP's menu bar>folder options>view tab. The boot.ini, ntldr, and NTDetect.com files will then be visible at the root of the XP primary. Those will need to be copied over to the root directory on the E 7 drive. From there you may not have to edit the boot.ini file as that already points to "rdisk(0)partition(1)".
The last step is to set the 7 drive as the default boot device in the bios. Once booted in 7 you can change which OS will be default as well with the EasyBCD tool in settings or through the msconfig>boot tab.
