You can change the day of the week, but as I mentioned in an earlier comment, DATEDIFF is not affected by DATEFIRST settings. I'll add a section about that into the article when I get a moment and can submit that and a few other changes.

Also, as Jeff mentioned, converting to a char/varchar and then back to datetime is considerably slower... but we're talking about fractions of a second per row, so if you're only doing a couple it's not a big deal. For instance, doing anything to GETDATE() to store in a variable... not gonna make much of a difference. But If you need to do it to the column of a table with a million rows, you'll definitely see the difference.