I have the same truck. Do these in order and you will see your truck come alive.
1. Remove mudflaps and side running boards.
2. If you have steel wheels, get them painted black.
3. Do PML(Poor Mans Lift). Lots of other posts just on that. See the DIY section.
4. Get better tires such as 31" mud A/T or 33"(you will need a 2" body lift.)
5. Do a google search for xterra parts and there are a few companies. 4x4parts.com is good.
6. If you have the extra money then you can get into customized metal front bumper, trail lights, rear tire carrier etc... these cost alot of money but make your truck look good.
Hope this helps. Some others will give other advice as well. Doing just #1 will make your city looking truck, trail worthy and alot higher.