Is the dress a high or round neck, or a "V" neck? If it's a high or round neck, a long necklace could be nice with it. If it's a "V" neck, soemthing that falls into the V is attractive.
If you make the necklace "flashy", keep the earrings simple. If you make the earrings "flashy" (ie-dangly chandelier style earrings that are so popular now) then keep the necklace simple, if you wear one at all.
If it's a high or round neck, consider a sparkly pin on your left upper chest(on your clavicle bone) to draw some brightness to oyur face without a necklace.