VBA help on the SaveAs method says the following formats are available:

ac2000_dwg
AutoCAD 2000 DWG (*.dwg)

ac2000_dxf
AutoCAD 2000 DXF (*.dxf)

ac2000_Template
AutoCAD 2000 Drawing Template File (*.dwt)

ac2004_dwg
AutoCAD 2004 DWG (*.dwg)

ac2004_dxf
AutoCAD 2004 DXF (*.dxf)

ac2004_Template
AutoCAD 2004 Drawing Template File (*.dwt)


-- Paul