The CSS property, Z-index is the property that defines the display order of HTML elements using relative or absolute positioning “position: relative; or position: absolute;”.
The value you give to Z-Index does not matter, but the value relatively to other elements Z-Index values will define which element comes in top of others.
The minimum value of Z-index is 0; it cannot be a negative number (but few old browsers versions are allowing negative values).
Z-Index maximum value is 2147483647, it’s a 32-bit value and it works for most modern browsers (IE6, IE7, IE8, Opera 9, Safari 4, Firefox 6, Firefox 3) except for Safari 3 which has a maximum Z-index value of 16777271.
Also when you set a Z-Index value superior to 2147483647 most modern browsers will use the value 2147483647.
So, I think it's safe to use values ranging from 0 to 2147483647.