My question was if I could do it with my current drives. I thought I seen somewhere you can do it with IDE or SATA drives and that they don't need to be identical but using identical ones was pretty common.
As far as why, I just wanted to try it, seems like a lot of people use it.
You might be able to rig somekind of software raid with the drives you have now but it would not be pretty and be somewhat useless.
Lots of people are making raid0 arrays (stripe) for their boot drives because it does increase transfer speeds considerably. To do that would require two identical drives hooked up to a raid controller and to have the raid controller configured to raid0 those two drives.