What kind of accuracy can I expect from my GPS?

10 - 20 meters (30 - 60 ft.) is typical, but there are a lot of variables. You should learn to check the GPS receiver's estimated position error value. Depending on the amount of sky visible, the configuration GPS satellites overhead and numerous other factors your accuracy at any given time could be much worse. (I've seen EPEs as high as 900m) With a WAAS capable receiver and a WAAS satellite in view you may get accuracies as good as a few meters.