I know you have heard some of the R&B recordings released in the last couple of years. How many of them can you accurately remember the melodies, and even the words to? Not many, right? This is because the melodies are all over the place!
Now, think of some songs that you really remember well-even better songs that you don't like that you remember well (like that irritating commercial jingle that you can't get out of your head) you will find that almost all of the notes fall within one octave, check it on a keyboard! Olive's 'You're not alone' pretty much takes the cake of recent dance releases on this score, the melody only takes up an interval of a minor sixth, that is, six notes of the scale the melody is made from, in its minor key.'Missing' by Everything But the Girl, which burned up the charts a couple of (or was that three?) years ago is another good example. The melody only takes up a ninth (the octave plus one note) in its key of A minor. This is why it's still easy to remember, was easy to sing, and sold 111 Gazillion copies!
Now contrast, if you know the melody, the US national anthem 'The Star-Spangled Banner'. Even if you "know it", you don't know it (most US citizens DO NOT remember all of the words and don't remember the melody; and with its octave-and-a-half range, no one but professionally trained singers can do it justice).
Do you want to sell 111 Gazillion copies, or be an orphan in your own country? Keep the melody simple, keep the range to an octave or less, and you can do the former, with confidence.