If you simply unplug the SATA hard drive, then stick the IDE drive in, then install W7 on the IDE to play with, then just reconnect the SATA and disconnect the IDE, you'll be able to go back to XP without any issues.
Better though just to backup everything from the SATA drive onto the IDE drive, install W7 onto the SATA drive, then copy it all back across if you ask me.