For AM2: I think there's an easy upgrade path for those that are conservative with money, but want to be in the upper tier (top 20%) all the time.
If you built with AM2 right now (for the cheap MBs and $110 X2 3800 for instance), or already have it, and except for constant video encoding or big screen play on just a couple of games, a low end dual core (3800 or 4200) are fine, or an opty if you like to overclock, then....
In a year you could upgrade to one of the low end quad cores from AMD via drop-in into your AM2 board, because....in another 6 or 12 months you buy an AM3 MB (skip the AM2+)....and another 6 or 12 months after that, finally an AM3 type cpu. This avoids buying too often, and when you get the AM3 board, at that point, your other component purchases could be future proofed at that level (this is why you'd get the AM3 board earlier than the cpu). etc.
This is a path that reduces costs, but keeps you reasonably up there with a powerful system.