I've had excellent results using EZPrints, www.ezprints.com. As far as DPI is concerned, I generally use 200 DPI and haven't had any problems. Some places will require higher DPI, but these are generally the more higher-end custom places. You should be able to get quality results using anything in the range of 150-300 DPI with the mass-market places like PrintRoom, EZPrints or Ofoto. Experiment to determine what is best for your needs.