I look at a platform as a unit: MB, CPU, RAM. Though it is good to leave room for RAM, GFX & HDD expansion, I (and I suspect most others) don't change core components like the CPU or MB. While future CPUs may fit the socket, their new tech will probably be limited by the chipset and RAM type. So it's built as a unit to stay a unit. If a platform performs a certain task today it will ALWAYS perform that task, maybe better with more RAM, faster HDDs etc., but if it's good, it will always be good. Same w/ bad.
My previous rig, P4 2.53, 1GB RAM, P4S8X was improved with a modern video card and more RAM and lives on as my wife's PC. The one before that, Socket A, is a file server. It's good to have a hand-me-down chain. Keep your old PC intact. Don't try to re-use your old PSU or HDD, buy current versions and you will be happier. Do your homework, like this post.
Purchasing a new PC is an important decision, buy what you like or you'll be forced to like what you buy.
Tony