You must use Xeons and it must be Xeons in the 5000 family

All socket 1366 Xeons in the 5000 family are not expensive.

An used E5620 4 core 2.4(2.53) ghz with 12 mb cache can be had for ~70$ here in Sweden.
X5650, X5660 and X5670s 6 cores have been sold for as little as 200$ but often go for 400+$ since these CPUs are what the people selling the E5620s are upgrading to.

Do they have to be matched pairs, or is it enough that they are the same model? I remember back in the day if you were building a dual p233 system they had to be from the same stepping.

I don't see the point of investing in a multi-processor system unless that's exactly what you need. Building with legacy multi-processors is both expensive in terms of the sum total invested in the equipment and the subsequent power use once to get everything up and running.

If this is for something basic like a Minecraft server, my recommendation is to purchase as many cores as you can on a single processor, as your power bill will make up the difference in cost.

You can pick up a Supermicro H8SGL-F fairly cheaply on ebay and, if you're really lucky, nab one with a processor in it too. There is one up on ebay with an 8-core Opteron for less than $300: http://www.ebay.com/itm/181265649319

Keep in mind that if you want to drop a 16-core Opteron into one of these you will need the bios to be updated. This means that you will have to confirm that it has the latest bios BEFORE you buy or make sure you buy one with a bundled processor so you can do the update yourself.

Xeons are nice but Opterons are the way to go if you want to build a cost-effective server. That's what I did and it's running great on CentOS 6.4.

Also note that most boards don't come with a heatsink and the ones that do run extremely loud. Here is a fantastic G34 heatsink that's nice and quiet:

http://www.newegg.com/Product/Product.aspx?It … N82E16835608023

snorg wrote:

Do they have to be matched pairs, or is it enough that they are the same model? I remember back in the day if you were building a dual p233 system they had to be from the same stepping.

You do not need a matched pair. With many boards the CPUs do not even need to be the same speed or have the same core count!
My EVGA boards accepts pretty much any combination. I have tried a E5620 4 core + X5670 6 core and X5650 + X5670.
On my boards the two CPUs can even use different multipliers independently* so both CPUs do not need to stick to the specs for the lower clocked CPU.
I would think they have to be in the same generation though so I would not try to mix a Xeon 55xx with a 56xx.

To be on the safe side and for practical reasons I would try to get two CPUs of the same model though.

*Only when default multiplier setting is set in the bios setup. I can not force the two CPUs to use different user specified multipliers. I can force both CPUs to use the same user specified multiplier but both CPUs need to support it.
I would imagine that boards without overclocking or multiplier options would work as my board does with default bios settings.