The ultimate choice is going to be what you feel confortable with and what you can afford. What resources from your P-III can be re-cycled into an upgrade, and how aggresive do you want to get with this upgrade. If you are coming over to the AMD side of the Force, you will be suprised where the XP1600+ with the right mobo and good quality DDR RAM can take you. A tweakable board ( I like ABIT's KR7-133R, my current board ) but you need TRUE OEM Memory - thats either Crucial or Samsung, and NOT some other makers PCB mounted with Micron or Samsung, they don't sell their premium chips to their compeditors - would you? Power suppy, again nothing under 300W, and if you can afford it a little over 400W is better. What do you plan to do with this upgrade configuration? gaming? some Vid's work better than others, no games - don't need as much punch! Get's to be a little bit like feeding a growing monster, the more you want to feed it the more you want to feed it. A Black Hole in Cyberspace that has a Tractor Beam on your wallet. List your parts, and figure out how to slay this dragon.
We come in PEACE ! Set phasers to KILL !