I have a home-built system with a Gigabyte MA74GM-S2 M/B & 2gb memory. My C: drive or drive #1is connected with an ATA and I have a second SATA drive # 0 H/D . My questionis since my Win 7 O/S is installed on the ATA drive can I designate the SATAdrive for the O/S?
No. Since the OS is installed on the PATA/EIDE drive, that's the OS or boot drive. The only way you can designate the SATA for the OS would be to re-install the OS on the SATA, or "clone" the OS from the PATA to the SATA drive and change the boot order in your BIOS Setup Menu. While the cloning programs (like Norton Ghost or Acronis True Image) would like us to believe this process is a no-brainer, they do not always work as planned. So make sure you have a good backup of all your important data before attempting this - and be ready for a complete re-install anyway.
That said, my recommendation would be to start with a fresh install of Windows on the SATA drive. This ensures Windows is setup properly with the new drive. Cloning has the potential of creating any old problems on to the new drive.
Windows Backup and Restore works for making an image and using it to image a new drive. From you question, it seems like the Windows folder is on the ATA drive, is that the case? But Windows Backup is limited in the size of replacement drives. Since it looks at all the partitioned space on a drive, you cannot use a new drive with less room than that amount of space.
What you need to remember is the primary drive set in the bios is very important, because that is where the boot files will go during an install. If you do not want any chance of having an install go where you do not expect, disconnect the other drive.