Backing up your apps to Astro is fine, but you won't get updates from the backed up apps. And really for what you're looking for the soft factory data reset is all you need.
So it will clear away all of the "junk" that may have accumulated?? Another reason I want to do this is hopefully I will get back the ability to send MMS's. And what about the SD card?? Anything that can be deleted that won't affect the phone.
It won't delete anything on the SD card. It will clear all your data so it will be just like when you first got your phone. You'll have to sign into gmail, google voice, facebook, etc. Many people have done this successfully to speed up a lagging Droid.
I've had to do it once because it was the only way I could figure out how to get my voice capabilities back. If you can't send MMS then by all means try it.
You may want to pull the battery first as someone else suggested. That might help and can't hurt. If you do pull the battery make sure you have it out for at least 30 seconds to allow time for the capacitors to discharge.