You may get better results in creating the full csv file by using .JetExportTable instead of using .ExportTable. See the programmers manual for the JetExportTable syntax.
I don't usually export to text files, so I'm not really certain if there are limitations there, off the top of my head.
But if you're using XL 2003 or earlier, the size of the exported csv file (assuming Monarch exports all of your text file) is your big problem, not how you ask Monarch to create it.
The minute you import the fresh csv file into Excel, you'll run into Excel's row limit.
Since you're not actually making any changes to the data file itself, you're really only renaming it, use FileCopy instead:
ought to do it for you.