New install of XP with ne HDD keeping existing HDD OS

Sorry could not fit my whole statment in the subject line. Here is the situation , I am going to be building a new rig sometime in the future and I have an existing install of XP on my current HDD. I plan on buying a new much larger HDD for a fresh install of XP with my new rig. Now I plan on keeping my old HDD but dont have anywhere to back up the data currently on it when I get my new rig.

Once I get the OS intalled on the new drive I want to move the stuff I want to keep from my old drive to the new one then reformat the old drive to clear it all up. The new HDD will be a SATA while my old one is IDE, should I wait to plug in my old IDE HDD until after I get my OS intsalled on the new HDD? Or can I plug them both in and set the SATA drive to be the boot drive in BIOS and then do what I need to do with my old drive after installing the OS on the new drive? And lastly if I chose the former and intstall my old IDE drive after the OS on the new HDD will the fact that I have an existing OS install on the IDE drive mess anything up on the boot up after I install the old drive to move what I need to move to the new drive?

Good deal thank. I was just worried there might be some conflict on the boot up since both drives have XP installed, I didn't want to have to dual boot set up or anything.