First look into getting a good suspension setup.
Some wider a-arms, shocks setup for your riding style and weight, antivibe stem (maybe taller if you are a taller person), steering damper, good tires. These mods will help you ride faster in the trails.
With money left over (if you buy some of the parts used you should have maybe 1000), look into a http://drivelineperformance.com/yamaha-banshee-392cc-big-bore-assassin-cylinder-kit/, this kit should boost your power some. Then you will need to upgrade the clutch maybe this http://drivelineperformance.com/yamaha-banshee-dune-trail-clutch-kit/