ok... short list:

Cams
throttle body
Exhaust
Cold air intake
remapped Computer
and misc. "top secret" parts

got a non-c/s xterra no where near 250hp... probably close to 200hp...

so, in addition to the above, I would assume you need to also think about:

headers
New pulley for SC
Water injection

and still I'm not sure your gonna get to 250hp