Question on Eternal Belt BuckleFollow

Hi guys, need a piece of advice here. I just recently got this gear Sash of Jordan http://www.wowhead.com/?item=44309.
Was wondering whether by adding the eternal belt buckle, will i get another slot or will it overide the current slot. Also can I equip it with red gems or I need to get the prismatic gem. Alittle worried cos I don;t want the current slot to be over ridden.

Kindly clarify my doubts

Thank you