Well, I have an X2 4200+ in a socket 939 motherboard, so any upgrade would be pretty fruitless. But you know what? It doesn't matter all that much as I generally get the upgrade itch roughly every 4 years- that's generally when the system starts to bog down some when doing my usual keep-a-lot-of-stuff-open work. Also, 4 years is about the time when there are a lot of improvements in speed and power of everything from the backplane on up front. I want to see a *real* increase in speed when I upgrade, and it takes about 4 years to get that 2-3x+ increase that is certainly noticeable in everything that is done on the computer.
I'll save my monitors, HDDs, case, and probably the CD and DVD burners. The PSU can stay if it will still work with new stuff and if it's big enough (500W should be, but...) The old mobo, CPU, RAM, and GPU would either sit in a box in a closet in case I or somebody else need them for something or they go to refill an even older box that a friend or parent has. My parents have some rather old machines and I highly doubt that they upgrade until I do again in about 2010. I'll just buy an inexpensive case and PSU and use the 120GB ATA HDD and optical that's in their current HP 1.8 Willy and put the old gear in it most likely. That Willy sure is poky- dang.