I sent you a msg via MSN, but I'll repeat it here.
Try taking out the 500GB and boot up the system with just the 320gb drives.
it should say degraded and then you can add the new drive to the RAID. After that point try and reconnect the 500GB and see if it shows the 500GB as a separate drive. If it does and windows doesn't boot up that might mean that the RAID has already overwritten the 500GB with RAID info. If that's the case you will need to reinstall Windows unfortunately. Let me know how it goes, good luck.
