I bought a broken xbox 360 in hopes of fixing it and modding it. The drive, a Samsung MS25, was flashed before I got it... I'm just worried that if i ever have to replace the drive, I will be totally screwed on account that the original firmware is not accessible to me. Will I still be able to retrieve the original Drive Key from this drive to put onto another drive?

The reason I am asking this is because I am considering buying another motherboard for the sake of having a backup to tinker with.

Ya, just dump the firmware off of the drive you got now. Will act as MS28 though, you are going to have to disable firmwguard. (Very easy, I know you can do it with MTKFlash with a VIA chipset or with dosflash with pretty much any chipset, the MTKFlash method is in Textbook 2008)