If you fold into rectangles, place them either on the plate, or to the left of the forks. Never place them underneath the forks. Choose a centerpiece that will set off your linens and dishes. Centerpieces shouldn't be too tall. You don't want your guest to have to crane their necks to see each other.
Artificial flowers are perfectly acceptable, and have the advantage of being reusable.
But don't think that a centerpiece has to be floral. Use your imagination. If you use candles, keep them lit until the last dinner guest has left the table. Candles should be placed next to the centerpiece. Formal dinners seem to demand the use of silver.