Use "with regard to" when making reference (as within the text of a business letter).
Another option is to just "RE: _______" at the top of the letter/e.mail and separate from the body.
"Regards" has a different meaning, and could be used as a way of ending a business-like correspondence. ("Regards, Lisa") It could also be used as in, "Give my regards to Broadway."
There's also using it as in, "I regard this as a serious matter."
And, just a note (and example) on a somewhat related matter: Regardless of what many people seem to believe, there is no such word as "irregardless".