Koroush over at TweakGuides walks you through the decisions he made duing the upgrade of his personal machine from a P4 2.6GHz to a dual-core A64. If you're going to be upgrading your own PC sometime soon, then you should give this article a read, because you're most likely going to be making the same decisions he did...

"I wanted the best I could afford, and I wanted to future-proof my system as much as possible. Finally, I just wanted a system which worked straight off the bat, without all those terrible incompatibility issues, bottlenecks and hardware glitches we keep hearing about from various setups. The results are in my Hardware Confusion article, and might help you make a better choice if you're looking to upgrade as well."