Sorry 100n3y, I rushed and ruined that whole reply, so it didn't really make sense.
Anyway, I thought you were doing a softroot from the SD card, which is what I've usually done. Whether it will work for you? I have as little idea as you do, but if your serial number begins with 1300, then it's not likely, if at all to work. It'll probably brick your nook, which you most probably don't want.
The 2.5.1 softroot package is basically the 1.5.0 Barnes & Noble software for the NOOK, but slightly modified to run as root and stuff like that. Don't know why it's called 2.5.1 either
What I've done, and I think others may have, I cannot confirm for any of them, but they might pitch in, is to softroot the nook, then just update it over-the-air (OTA), which seems to work...no guarantees there though, I've only tried it once.
So now I'm running firmware 1.7 with all the benefits of the softroot package. From what I gathered on the nookdevs site, there really isn't much documentation on that unfortunately. You could always try the IRC chatroom for Nookdevs, or post on their 'forum'.
Sorry I can't be of more help
