We can't use the Express Tools version as it ignores the "Save as earlier version" setting and plenty of our clients are on earlier versions.
This is where eTransmit comes in REAAAAALLLY handy. Just set up a transmittal to save as 2000 or 2004, grab all of the files, push a button and *poof* you've got your drawings (and all supporting drawings) saved as an earlier version.
DWGTrueView 2008 can convert multiple directories at once. You can also fiddle with a host of settings at the same time such as setting plotter to "none", replacing page setups, binding xrefs, plus other stuff. You can overwrite the existing files, or send 'em to a different directory