Iis – Which CPU configuration for new servers (IIS7 / Windows Server 2008)

For the AD/DNS server for that few users, either will more than do.

For the SQL server, then I would go for the quad core [unless it will only be running single queries most of the time (i.e. not large multi-user systems) in which case the higher speed of each core might be more beneficial than the extra cores] all other things being equal. "All other things" being mainly RAM (you want as much as you can shove in there) and IO (a good IO sub-system is a must for an active SQL server).

First off, a 2.6GHz processor is not a 2.6GHz processor if they're from different generations. You're correct in thinking twice about that. This has been true for a long time now (at least since the 486 / Pentium days), and so it's important to point out to your engineer just how wrong the Megahertz Myth is. Especially given the massive performance improvements i7 based chips offer over Core / Core2 based ones at the same clock speed.

That being said, that's not my first concern with this plan. My first concern is that used servers will have a significantly reduced operational life that a new server, since you don't know how it was previously used, under what conditions, or what'll happen to it in transit on the way to you. Generally speaking, for production systems, reliability should always take precedence to performance, since it'll cost you way more to fix a dead production server than to upgrade a server that's too slow.

My feeling is that the price difference would have to be very, very substantial to even want to look at doing this, and that if you're buying used, you'll want to redundantly cluster them just to be safe.