advice on GPS for scientific use

I need advice on a good handheld/portable GPS unit for
scientific/environmental use. Primary use would be to locate sampling
locations, so horizontal accuracy is important. elevation accuracy
would be nice, but it seems like it isn't possible to get good
vertical accuracy. Cost is also important- cheaper is better (of
course).

Does anybody have suggestions on models that would work, or good
places to find information?

josh