Thanks to this forum...
Did some much needed maintenance work to my wife's '00 Xterra. It has the 3.3v6 with 155k on it.
I did the following:
-Changed Oil/Filter (Full synthetic w/a K&N Filter)
-Changed Fuel Filter (I believe it was the OEM one..eek)
-Changed Air Filter
-Did the intake mod (Thanks to soccerbrace to the tutorial)
-Seafoamed the gas tank and ran some through a vacuum line.
-Changed spark plugs (distributor and plug wires are new, don't know why the plugs weren't changed)
I'm very happy with the results. Before, she averaged 14-15mpg mixed highway and city driving, and she drives like a nut. This past week she averaged 17.5mpg with her same commute, can't beat that.
I just wanted to thank everyone here for all the helpful tips and information that is posted every day. In about a year or so, it will be time for her to get a new car (she's been driving her X since she got her license 8 years ago) and I'm hoping she chooses a new X.