DAVID
Here is @dam_catching_potter's explanation:
Five hundred begins it, five hundred ends it,
D = Roman numeral for 500
Five in the middle is seen;
V = Roman numeral for 5
First of all letters, the first of all figures,
A = the first letter
I = the first figure (number - in Roman numerals)
Take up their stations between.
Join all together, and then you will bring
Before you the name of an eminent king.
DAVID
@PiMaster solved the riddle first! Great job!