Don't get me wrong, I'm not talking about any dirty tricks or paying OEMs to include their products or the like. I'm talking about Sockets and processor support on motherboards. One thing AMD ever since ditching the Socket 7 CPU models has been famous for is a different socket for different product generations, and even switching in the middle of a product's life (S754->S939). I'm sure there are architectural reasons behind these changes and why are they needed, however Intel seems to have managed pretty well and for some time now they've been using the LGA775 socket in a wide range of products (from Pentium 4's to their latest Quad Cores, that sums up 4 different generations [P4, Pentium D, Core 2 Duo, Quad]), from the looks of it this has enabled Intel to have a more stable "mixed" environment and to empower their users to be able to upgrade and be forward compatible. For instance you could get a motherboard with a certain CPU and get a newer model down the road and the motherboard will accept that. Maybe without all the features, giving the user a more gradual upgrade path, and upgrade the motherboard down the road to a newer one which makes better use of the CPU.
On the AMD side, it seemed it was going to be the case with the introduction of the AM2 socket, but it wasn't until this socket that they attempted this (that I can remember, anyway), as before this there ware Socket A for the XP line of products, S754 for the first generation Athlon64, S939 for the "second generation" of Athlon64 (were these the "New Castle arch?") and finally AM2 for the dual core CPUs. With Spider they are also inroducing yet another socket type, AM3, apparently pin-identical to AM2, but with fundamental changes, especially to Hyper Transport (and memory & IRQ management, I believe). This is good, as at least they have announced that Phaeton CPUs will be compatible with current AM2 sockets and AM3.
The situation about different sockets for different products has had the effect that AMD upgrades seem to be "more expensive" than Intel's as usually a change in CPU also implies a change in socket and thus motherboard. While Intel users have been able to upgrade more gradually for some time now. With the recent AM2->AM3 it would appear as if AMD has adopted a similar strategy, I hope they indeed have... It would look like for AMD socket configuration represents something analogous to Linux's "kernel API", i.e they don't have a "stable one"
Ehm, clear case of "no" in regards of intel updates. In general you will need a new motherboard if you upgrade your cpu on an intel hardwarebase because the new cpu does need a little different power supply or a higher fsb, so that the old boards are often not compatible even though they have the same socket.
Hmm, Interesting. I've been for the last several years embedded into AMD systems, until recently that I'm on the verge to upgrade is that I am looking at the Intel side of things... I see you point, but I got confirmation from a few friends with Intel hardware since Prescott, that point to what you say, however with Intel motherboards you can at least upgrade from one gen to the next in a "current" motherboard more often than not. It was actually the case with one of them who upgraded from Prescott to Pentium D on the same board, but had to change motherboard for C2D, which he plans to use at least initially for his C2Q.
And with AMD that was possible, too. Do you remember the switch from "normal" Athlong 64 (unsing socket 939 (that is DDR1 ram)) to Athlong 64 X2? The first ones were socket 939, too. At AMD you basically have to switch the mainboards to get a new ram interface since the memory controller is embedded in the CPU, not in the mainboard. So difference between 754 and 939 was that 939 had a dual-channel memory interface where 754 was meant from the beginning onwards as lowcost solution with a single channel interface. When DDR2 support was introduced AMD had to switch the socket over to AM2. And that is basically where we currently still are. The problems with the Phenom and many motherboards is basically the board vendors fault since they would have to provide a BIOS update to make them work. But it looks like many won't do so (probably they want to sell new boards...).
In short I would say that it is basically the same regarding upgrading when you compare AMD and Intel. When there are real differences you will need a new board for both sides. Of course updates in one generation are basically always possible. Problems arise when new voltage stuff is required and/or the frontsidebus has to increase.