While Raid 1 (mirroring) is great in the event of a physical drive failure. It will not save you in the event of an OS failure.
Now compund the fact that he would be creating the raid set at the OS level instead of at the hardware level. This causes an automatic performance hit using the OS to perform a raid function. And if you are using IDE drives tack on an extra penalty hit.
Also with Raid, (any level), make sure that you are using disks my the same manufacturer. prefabably the same make/model/size. Reason being is that different manufactures have different block sizes that they use. Raid is a testy creature and you may get some unexpected results down the road when mixing different drives.