you have a 2002, so your xterra is somewhat kind of new. if you really feel you didn't get what you really wanted originally (being the supercharger, 4wd, etc) you should take your chance now and trade it in for what you want. the longer you wait, the older and higher-mileaged your xterra will be.
i don't know anyhting about the aftermarket s/c that's been mentioned above, but with the factory s/c, it's a noticable difference but don't think it'll transform your xterra into something it's not already.
if you want to talk about trading in your xterra and getting something different, consider getting 4wd. that's something (if you offroad) that will make a much greater difference than a supercharger. 4wd will open your xterra up to more opportunities and options as an suv. with the factory s/c, let's be honest, it takes more than 30 hp to transform a 4000 lb truck.
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