As zacster said, I'll also assume the OEM brakes are Tektros and, as he further said, they are decent calipers fitted with poor to mediocre pads.
Cheapest upgrade: Replace the OEM pads with Kool Stop Salmon pads. The Tektros should have Shimano-compatible pad holders so you will need two pair of Kool Stop "Dura- type" pads.
Next cheapest: Replace the brake calipers with either 105 or Ultegras depending on your choice and budget. The Ultegra calipers are slightly lighter and a bit prettier but won't stop you any better and do cost more. Shimano's pads are pretty good but changing them out for Kool Stops is sort of the icing on the cake.
Extravagant upgrade: Go for Dura Ace or even Zero Gravities calipers and blow the budget completely. You will save a bit more weight and greatly increase the bling factor but they won't stop you any better.