[CarSuperfreak]

either of those problems could just be gage related. However, the 3800 engines (both supercharged and natuarally aspirated) consistently require new fuel pumps on higher mileage cars. If the car runs ok, I wouldnt worry too much about the fuel gage, just watch how many miles youve got between fill-ups.
As for the temp gage, I would first get your coolant checked. In the 96, you should have the orange dex-cool coolant, which GM says is good for 5 yrs or 150K. not so. they ran into all kinds of problems with the coolant 'gelling up' and clogging coolant passages and lines. I would first get the coolant checked (and most likely changed) and then have the thermostat checked out.