It is a chapter whether it is titled or numbered; there's no term exclusively for a chapter with a title. If you really need to distinguish between them, you might call them titled chapters.
NOAD: a main division of a book, typically with a number or title.
Random House Dictionary (via Dictionary.com): a main division of a book, treatise, or the like, usually bearing a number or title.
Collins English Dictionary (via Dictionary.com): a division of a written work, esp a narrative, usually titled or numbered
The American Heritage Dictionary (via TheFreeDictionary.com): One of the main divisions of a relatively lengthy piece of writing, such as a book, that is usually numbered or titled.