AXP 1800+ is fine right now, most likely it'll last you at least another year. You'll probably be able to use it until Hammer 0.09u or Prescott. By then you'll be able ot get a XP4400+ or a P4 4.0GHz... Until then you should be fine. Really no point in upgrading until then unless you have loads of money to burn.
My firewall tastes like burning.