Installing a second harddrive and video card

I just built my own PC recently, and I was thinking of using my previous harddrive as a secondary on my new computer. But I'm not really sure how to change the CMOS to make the new HDD as the primary one, and the previous one as the slave. How exactly do I do this?

Also, I have Windows XP on my previous HDD, and Windows 7 64 Bit Ultimate on my new HDD. If I were to use both harddrives in the same computer, would the different OS in both HDD effect each other? If not, would I be able to use the files in the Windows XP HDD, or would I have to format it?

In your BIOS you can specify which hard drive to boot from. Its a little different for every motherboard. Its usually listed in order of top to bottom so just put the new hard drive on top of the list. You can likely manipulate the order with page up and page down.

Yes, one Windows installation can see another installation. Your Windows 7 can manipulate files on the your Windows XP and vice-versa.