The 2005 Pathfinder has mileage estimated at 15-23 MPG. The 2005 Xterra will have the same motor, minus a little horsepower and torque.
The 2004 Xterra is estimated to have 17-20, although you will easily find people on this board who only get about 15 MPG, myself included.
Simply put, you're buying an suv so don't expect Civic mileage. Just to keep yourself from disapointment, start expecting mileage in the high-teens at best.
Personally I would wait for the 05s. Whether or not they offer any incentives for it, it is worth the wait and the extra money. It is a jump of 70 HP and 70 Lbs of torque. There are too many new things to list here, but go to
www.nissanusa.com to check up on it.