RAID is a poor idea unless you really need it. With very mismatched drive sizes you will end up with the larger drive having one Partition involved in a RAID array, plus a bunch of empty space you need to use to create another Partition for data storage. Then there's the possibility that your older drive will fail much sooner than the new one, ruining your RADI array and (in the case of RAID0) losing all your data.
Better choice, I think, is to migrate everything from old to new drive. If you don't have it already, go to the WDC website and download (free) and install their Acronis True Image WD Edition software. Run it to clone old to new, making the new drive use ALL of the 640 GB, not just duplicate the old 160 GB Partition. Make sure it is set as bootable (if you have any choice on this) and uses the NTFS file system. When you're done cloning, my practice is to disconnect the SATA data cable from the old drive and plug that into the new drive (removing the new drive's temporary data cable). That way the new drive is now connected to the SAME SATA port that always had your boot drive on it. I'd leave the old SATA drive completely disconnected for now (can leave mounted inside the case) as a good backup (up to the cloning time) you can use if the new system has a problem. Once you're happy the new drive is doing everything right, reconnect the old SATA and destroy all its old Partitions. Then create a new Primary Partition (all of disk, NOT bootable), format is to NTFS, and use it as a data storage unit.
Another possibility: buy an external drive enclosure and mount the old drive in it. Use it to make periodic backups of your main drive, then disconnect it when not in use so it is isolated from the computer system and from electrical supplies, including any bad surges or lightning strikes. You can even move it offsite when not making backups. One of the easiest ways to make that backup (if it will fit) is to use that same Acronis True Image WD Edition software to clone the main drive to the external drive - then you get everything at one shot.
In choosing an external enclosure, make sure you get one with a SATA II internal interface to the drive itself. For the interface to your comp, check what your comp has for this. If you have eSATA it is the fastest commonly in use. Firewire 400 (aka IEEE 1394a) is about the same. USB2 is a little slower, but is found almost everywhere. Mine has both USB2 and eSATA.