SUSE :: Renaming Multiple Files - Adding Date / Time Stamp In Name

I am trying to rename multiple files in a directory and add the Date/Time stamp to the file name.

Example:
file1.txt
file2.txt
file3.txt

Renamed to
2009_02_01_21.txt
2009_02_02_21.txt
2009_02_03_21.txt

I am able to accomplish this using this command:
mv file1.txt "`date +%Y_%m%d_%H`".txt

But I want to automate this task, I have many files to process and I dont want to do them one by one.

I already tried this but it does not work
mv *.txt "`date +%Y_%m%d_%H`".txt