Submitted by OffroadX:
It's been ages since I did it, and I'm pretty fuzzy on it in general. For the Xterra/Frontier VG33, options are:
110 Amp from the VG30E Quest (up to and including 1998 model year)
125 Amp from the VG33E Quest (1999-2003)
The 125 Amp one is a simple drop-in, but the pulley is a little larger and the original belt is a pain to get over it. Re-using an old belt isn't too bad, but a new belt is nearly impossible. You can step up to a longer belt, the part numbers reflect the length and any store should be able to steer you to one an inch or two longer.
Better to swap your original pulley in place instead. I think it's a direct swap, but it may be that the pulley hits the raised area of the case around the shaft exit before the nut can tighten the pulley onto the shaft. If so, you can easily grind down a few millimeters from the case around the shaft exit so that the pulley bottoms out on the shaft before it hits the case. This grinding might not be needed with the 125 amp alternators at all, just the 110 amp ones, I don't recall. Either way, be prepared to keep your original pulley and grind some of the case.
Use a "strap wrench" to hold the pulley, and an impact gun on the nut. There may be other ways, but that's the best that I know of.
I don't know the specifics of replacing it, but it's advised to upgrade the main fuseable link to handle the additional amperage.
If you've done this swap and have additional info, please PM Desert Rat with it so we can add it to the How-to.