i have some 5.25 from another car i had, and i was wondering if it would fit on the xterra. Do i need a frame adapter??

What year is your X? The 2000-04 models have 6.5" components in the front and 6x9" in the rear. Im not sure about the newer ones. If you want to keep your 5.25's then there are spacers that allow them to fit if not you can purchase some abs plastic board and modify it to fit in the doors.