Should I upgrade my old 2002 Diamondback Coil mountain bike? or should I invest in buying a new bike?

My 2002 diamondback coil has been a very solid bike for me, but now that I'm finally getting into mountain biking, I want a little more than what I have now. The bike is unfortunately not compatible with disk brakes (no mounts on rear triangle or fork), but the frame is very well built and strong. I just don't know whether I should spend a bit and upgrade my trusty old steed, or shell out some real cash and buy a brand new bike.