Compatibility can be a bear. Especially when you are just scrapping together server and desktop parts and trying to get a good deal while also getting good performance. Many people are drawn in by cheaper ECC memory or Xeon processor that are cheaper than their desktop counterparts. This is a very interesting option for people looking just to set up a home lab or a high-performance gaming rig. As of writing, the 8 Core E5-2670 can be had for around $70. There aren't even any options with 8 cores on the desktop side under $500 dollars, let alone $100. The biggest hurdle though is if it will work in your motherboard. If you would like a general guide showing which desktop chipset might have a chance of running a Xeon processor or vice versa you can check out my General Motherboard Compatibility Guide, but in this article, I am going to focus on some bizarre quirks with Intel's C200 series of motherboards.
The first thing I am going to talk about is what has changed. It used to be that the memory controller was a component on the motherboard inside of the chipset, but in recent years, starting with Nehalem, the first generation with the i7 naming scheme, moved the memory controller onto the processor itself. Previously you used to have to look at the chipset of the motherboard to see if it could handle a certain memory capacity or speed and when they moved the memory controller onto the processor you would think that then you would have to look at the processor's compatibility with the memory. This is true, but there is a catch. You still need to look at the motherboard's chipset because it still plays a part in compatibility. The most major thing to note is that while the C200 series chipsets all have desktop equivalents that seem like they should be the same, they are actually slightly different in the memory category. As you would expect they support ECC memory. Since the chipset is only designed for workstations and not servers they use Unbuffered ECC memory(If you are confused about memory type then check out my post on memory compatibility) instead of Registered ECC memory.
Generally, people think that in order to run ECC memory you need a Xeon processor, but things get confusing when you have HP releasing products like their Proliant MicroServer which comes with a Celeron a C200 Series Chipset and Unbufffered ECC memory.
So what gives? How is it that they put a Celeron with ECC memory? Well, It turns out that Intel left the ECC functionality on on their lower end parts like the Celerons, Pentiums and i3's for some unknown reason. This is interesting because it allows OEM's to make really cheap server systems because they do not have to use the more expensive Xeon processors. The downfall of this is that it make it very confusing to upgrade. You are capped at best with an i3. That is probably plenty fine for many tasks but you need to be aware that if you put an i5 or i7 in it isn't going to work.
One thing you might be thinking now is, Hey, the memory controller is on the i5 and i7 now so if I put those in there then I just need to use non-ECC memory. Unfortunately, as I said earlier, the compatibility is still linked to the chipset in some way so even though your i5 or i7 you want to put in there can support non-ECC memory, the C200 series chipsets cannot.
...Or can it? There is one exception to this rule. It seems that Intel has added non-ECC support to any C200 series chipset ending in 6. e.g. c206, c216, c226. This is quite peculiar because it seems like all of the chipsets are mostly the same, give or take a couple I/O options and PCIe lanes. What this means though is that if you have a C2X6 series chipset then you can use an i5 or i7, but since they do not have ECC support you will need to switch over to non-ECC memory. Theis makes the C206, C216, C226 chipset motherboards turn out to be some of the most versatile motherboards that have ever been released. Check out all of the possible CPU's that you can use for a C206 based motherboard like the Asus P8B WS . You can use Celerons, Pentiums, i3's, i5's, i7's and Xeon E3's From both Sandy Bridge and Ivy Bridge generations given that you have the correct memory for your processor. You could start with a system featuring a measly Celeron with ECC memory and then decide you want more Compute power and switch to an i7 with non-ECC memory and then if you determine that you need the ECC memory again you can switch over to a Xeon for the high computing power and ECC memory.
The C200 series is definitely confusing, but I believe that it is a very good deal either way because the variants ending in 6 have immense compatibility potential while the non-6 variants give great value and leave the possibility of Xeon upgrades down the road. You just have to remember that the chipset as well as the processor both must support the kind of memory that you are looking to run.