rsdigi -
you'll never get good sound from factory speakers. xterra's take a true 6" speaker up front, last I checked. Check out JL's VR600
http://www.jlaudio.com/stealthbox/xterra.html (towards the bottom)
You can find them on ebay for less then $100
Also, no head unit will power aftermarket speakers the way an outboard amp will. Consider a 2x or 4x amp to power your new speakers...it'll be money well spent.
Personally, I got myself a PPI PCX440 (4x40) for my internal 4 speakers. It was $$$, but I'll never need another amp again. Already made it through two other vehicles and about to go in the X. You can find the PCX440 for $150 refurb'd on ebay.