I can get my 2 Opteron 250s to 2592mhz stable (216 * 12) on air alone. All with stock heatsinks and some cheap 6000 rpm fans I found lying around. I also overclocked my 6800 U to 450/1200. I have crappy RAM so I set up a CPU/24 ratio on it, and it seems to be working well.
Overclocking Xeons/Opterons is basically like overclocking a 1 CPU rig, except that you're limited by the overclocking power of your worst CPU.
I think I got unlucky with my 3.06/533 Xeons. I'm only able to bump up the FSB on them (and actually lose 66mhz in doing so). Any attempts to run at 3.2 or 3.333 GHz generally fail within a day.