As you have two different HDDs, the best practice is to install the different OSs in two different physical drives.
Firsr, unplug the SATA and power cables of the HDD containing C drive (XP installed). Then Install Windows 7 in the other one. After installation is completed, reconnect the HDD previously remover. Make it sure that before opening the case, you are unplugging the power source.
Generally, the boot menu key for Gigabyte is F12. While power on the computer, press and hold F12, you will get the list of disks to boot from. Just select which one you need using the scroll arrow keys, and hit enter.
If , by any chance you want to have a dual boot, install EasyBCD in windows 7, and add a boot entry for XP using it.