I bought an Opteron 165 with the intention of OC'ing it and I've just never gotten around to it. I didn't want to be without my system long enough to test it for those days I would need. As a matter of fact, I was considering building a Conroe system just so I could OC this one (supposedly, I was shipped an above-average stepping but who knows).
Anyway, the Opteron has been really nice. I have experienced some of the dual-core issues and I don't love alt-tabbing out of Madden to set the core affinity, but when I do that, it guarantees me flawless play. It's an XP limitation from what I've heard. Only Server 2k3 optimizes the dual-cores properly (realize at this point I'm talking out of my ass and regurgetating something I read here by someone who sounded much smarter than me but it made sense and nobody disagreed with him). I have found myself mentioning Vista quite a bit recently but I seriously hope it addresses these issues.
The hardware has been out forever. I recall having to decide between the newly released A64's or the 3.0 s478 Prescott. Went P4 and didn't really regret it since 64bit still isn't really supported (again, Vista - though did I take a beating in performance...that P4 was outrageously inefficient). So for now, you are right about Dual-Core. There is no NEED, but soon there will be perks. Regardless, the thing about the Opterons that I have not heard as much about from the Athlon's is that you get alot of free MHz. Most get you to 2.4-2.6 on air with no increase in voltage. Bang for buck, this is a monster. If I'm not mistaken, at one point, they were hard to come by because of their reputation (David Grahm was having a hard time getting his hands on them - though again this could just be me mincing words). I say Opteron 146 and OC it as far as air will take you. The theory behind the Opty's is that since they are server/workstation chips, they are made from better 'stuff' (I sure hope I remember this stuff correctly so I don't get hammered) or cut from a better portion of the 'sheet', so to speak. Thus, they respond well to OC'ing. It was an OC'ers dream working with them from what I've read. A respected OC'er said something to the effect of 'Thank you, AMD, for the s939 Opterons. The greatest and most rewarding over-clocking experience of my life!'
Kinda makes you wonder if AMD could find a niche in the wake of Conroe based on 'free mhz'. Anyway, sorry to be long-winded but since you hadn't heard much about Opterons, I thought I would give you some perspective (that I hope I didn't get wrong).
Good luck and keep us notified on what you decide!