Web-server – Dell PowerEdge 1750/1850 using *SATA*

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.

Yes, the Dell drives are special -- they cost a pile more, have a Dell warranty, and your rep probably gets a commission out of selling them, all of which you don't get if you don't buy Dell hard drives. (Whether those are benefits or drawbacks I leave to your own best judgment). For the record, we run aftermarket drives in our R410s (mostly SAS, I think) without a problem, using the same trick (buy cheapo drives, remove the caddies, use drives for target practice on the clay pigeon range).