Cool Tom, how are those Toomeys? :yelrotflmao: Sorry, couldn't resist.
Anyway, a Banshee probably isn't the best choice for an MX bike if you are racing competitively. Although, it is a definately one of the most fun bikes to ride if your just a weekend warrior, or just having fun.
The mods that will give you the most bang for the buck are the exhust and jetting(Many choices for MX, Pro Circuits, Paul Turner, FMF, Toomeys, just to name a few), tires (if your still running stock, ditch 'em), and going one tooth lower on the front gear. These were the first 3 mods I did before I started going wild, at some point in the near future, I would also figure adding +2+1 arms, and new front shocks, this gets a little more pricey, but is well well worth it. This is from my own experience, I don't race, but I do ride trails and MX for fun.