Your method is right . Unplug the Disk0 (containing C drive), Install XP in another one. So the XP bootloader will not be installed in Windows 7 MBR .
Later you may apply a dual boot setting using EasyBCD, as you stated, but I would suggest you to not do that. You are using Gigabyte motherboard, where the boot menu key is F12. Press and hold F12 when the motherboard screen will be shown after power on, you will get the list the available disks to boot from (including all your Hard disks and DVD drives). Use up/down arrow keys to select which disk to boot from, and hit enter.
As you have two disks to boot from, you are to choose which one to boot by default, I mean if you dont press F12. Enter into BIOS, and set boot priority properly, better set boot from DVD drive first, Win 7 HD second, and XP HD third.
I am saying you to not set up a dual boot environment, coz if any of the OS installation gets corrupted, it will affect the other one under a dual boot system. So better to install to OSs in two different HDs, and use the boot menu key to choose one to boot from, when needed.