DYI steps please - 97 Pathfinder Oxygen Sensor and Throtle Position Sensor

Thanks for the replies.
Changing the spark plugs went alright except the last one at the back of the engine which was a bit hard to reach.
I read in the Chilton manual (which I borrowed from the library, did not know they kept those till yesterday) that the pre-cat oxygen sensors are the ones used to determine the fuel-air mixtures, hence why I wanted to change them. @underling: you are right. I can see the front O2 sensors (in the hood) and especially the driver side seems tricky to get to. Has anybody tried to change the pre-cats that themselves?

The TPS is from an error code (Actually two codes with same explanation). I have ordered the parts for that as well. Will see how that goes and report.

Report:
I changed the spark plugs and the air filter a couple days ago.
I changed the Throttle Position Sensor myself (because of a code), which seemed fairly easy to do, and is plainly accessible. Adjusting it might be a bit tricky. I hope I have got it right.
I changed the front two oxygen sensors. I bought the parts from Murray's Auto Parts, which matched Autozone's price and gave me 5% on top of that. I took them to my mechanic which charged me for an hour worth of labor. The front two are indeed tricky and hard to reach, so I am glad I took it to the shop.
I filled up my tank this morning, and am waiting to see if there is any improvement.
Thanks for your help.

Report 1:
I drove my first 80 miles (entirely city driving, with stop and go) after the maintainaces above (and a 33 psi tire adjustment). My milage now is ~15 (14.83 MPG to be exact). That is about a 34% improvment from the previous 11 or 12 I got with it. I will report on mostly highway driving later.