I've read both your posts and am still confused, try to write more intellegently next time.
I *think* you are saying you have an IDE drive now with a running system and you want to add the SATA drive to it, but transfer the OS over to the SATA, then use the IDE for extra storage.
If that is right, then Tedster is correct and your best option is to clone the 40 gig onto the 120. There might even be a utility that came with your hard drive just for this purpose. If not, Maxtor has one in their PowerMax software (free) that should work. Although I don't see why your 40 would have to be erased - then again if its cloned you wouldn't need anything on it anyway since it all got copied to the 120.
Your other option is to run the File and Settings Transfer Wizard on your running system, saving the files it creates somewhere. Then reinstall the OS to the SATA one and run the Wizard to import back the files that were created earlier.
One issue you will likely run in to is your system is going to want to boot from that IDE drive because that is how your BIOS is set up. You will have to change that so it boots off your SATA drive. Depending on what approach you take and your SATA controller you may need drivers for the SATA drive before you can do anything with it.