This same exact thing JUST happened to me.
It took me downloading a few different SBFs before I found one that wasn't broken or corrupt. Just keep trying. Flashing it in RSDLite is the Easiest Thing Ever once you find a GOOD SBF.
Search for "Droid X Full .340 SBF" on Google.
Be patient. The downloads take about 20-60 minutes depending on servers, and the flashing process can take about 10 minutes.
The first time you boot up your phone, it's going to be just like the first time you took it out of the box. It's going to take up to 10 minutes for the phone to boot up. Again, be patient.
Do Not, For Any Reason, At Any Time During Flashing, PULL THE BATTERY. THIS WILL MESS YOUR PHONE UP ROYALLY.
But yeah... you'll get it. It's a trial and error process. And that's one of the things we sign up for when we get involved with the Mod/Dev Community.