Powder Cocaine Mandatory Minimum Sentencing Act of 1998 - Amends the Controlled Substances Act to decrease (from five kilograms to 500 grams, and from 500 to 50 grams) the amounts of a mixture or substance containing a detectable amount of coca leaves, cocaine, ecgonine, or compounds thereof (cocaine powder) the manufacture, distribution, or possession of which is punishable by specified penalties under the Act. Directs the U.S. Sentencing Commission to promulgate or amend guidelines to reflect the amendment made by this Act.