I take it my car, a 99 SLE, has two O2 sensors..http://www.gmpart.com said that I need part #25312207 for the one after the cat, and part #25312203 for the exhaust manifold. Does this sound right? I need to know before putting 100 bucks into it. Thanks!

Also, I need to be certain that the O2 sensors are the only thing that would cause my car to drop to 16 MPG city and 22 HWY. Is there anything else that might cause it? My car runs excellent..no stutters, problems whatsoever. Thanks.

the first O2 sensor is the only one that will affect mileage. The second one only reports things like if the cats isn't functioning correctly

Absolutely. Don't change the second one for mileage. Only if you're throwing an emissions light.

Do the one in the manifold if you suspect an O2 Mileage related problem.