In terms of the 1st Gen Xterra, there isn't much to be gained by swapping out chips. The low compression ratio, small displacement and mild camshafts in the 3.3L's are not a good foundation for performance gains from a chip flash or programmer.
Chips and programmers see the greatest benefits in large V6's, V8's and big diesel engines. Essentially, those aftermarket chips allow for performance gains via timing advances, however, you'll have to run high octane fuel to realize it.
If I understand correctly, there is a chip available for the 2nd Gen Xterra and Fronty but the timing advance is only 2 degrees. Personally, it's not worth it unless you can get a 4 degree timing advance and run 93 octane. Then again, the 2nd Gen X 4.0L V6 makes 260HP/280TQ on 87 octane. That's on par or exceeds the performance of a stock 302 V8.