RAID Mirror with difference HD types
I don't see why not, so long as the raid controller is capable of using them in RAID 1: if they both run on a SATA controller, and it can do RAID 1 on those channels, yes; if one runs on a PATA controller, and the other on a SATA controller, and the motherboard can't do RAID 1 because they are on different controllers, then no.
But, it isn't recommended due to the vast differences between the two drives, which will result in slower speeds. It's generally recommended to use like drives when setting up a raid.