I noticed that the running lights on my car (headlights, dash, etc) dim when my car revs past ~3700RPM? Is that the battery or Alternator?

Any Ideas?
Derek

1999 Ford Contour SE (Black) S&B Powerstack Air Filter Brullen Cat-Back Exhaust System (Loud Setup) Sylvania SilverStar Headlight Bulbs MSN Messenger: ddasilva99@hotmail.com