Xterra is a tough machine...a good truck but not the best of gas consumption in its class if u read comparative results.
i would not spend money getting rims for an SUV like Xterra. Its a total waste because Xterra was designed with only work in mind...its not the likes of Murano, FX35 or the RX330 whom ppl pimp and stuff. Its tougher than all of these and is more practical.
I would get flatter and wider baseboards, get a good DVD entertainment system inside, GPS and perhaps...give it a full detail with a wax job. That will go longer than getting rims.
If u really wanna go for it...get new tires instead.