Dual-Core K8 processor will last you a long time. So will the 7900GTO (essentially a GTX with slower memory clocks. Bring it to 7900GTX specs and you have arguably one of the 3 best performance cards in the market).
What applications do you use? What games do you play? At what resolution?
Do you demand to run all your games at maximum res, maximum quality or are you fine with dropping a few sliders to medium and stuff?
For Two years, you will still have a mid-high gaming & content creation system. Relax.