Ok, this is kind of a complicated question.
Basically, when I build a new machine (Usually every 2-3 years or so), I try to build something that has lots of room for upgrades. I built this current computer almost exactly a year ago, but because of what I chose in terms of things like a motherboard (I am running a MSI P6N SLI), I have lots of room for upgrades. For a processor, I just chose a Core 2 4300, knowing that I would upgrade it to another LGA 775 processor later. I have a 7900 GT OC, which is still a decent card, although not DirectX 10, but like the processor I plan I upgrading it later.
Really, what it ends up being all about is what are you wanting to do with your machine.