Since you copied to me specifically... personally, I would prefer dual Opterons for web servers and have advocated that option. There are pros and cons to each option, so it's not an obvious choice either way and not something anyone is likely to get really unhappy about.
Pro-dual something:
*Less port and rack space use per box.
*One dual Opteron was tried and did well, hasn't been properly benchmarked so views on how well differ.
Con-dual something:
*We have a crude ping-type load balancing system at present and the single dual box initially grabbed a disproportionate amount of traffic until its priority was adjusted.
*Filling rack with dual CPU 1U boxes is sometimes said to be unadvisable for heat/power reasons.
*We had single CPU P4s initially to get plenty of boxes so losing one single box wouldn't do much harm. Inertia favors buying more of the same, as does the load balancing issue.
*A bit more expensive per CPU. Whether that's worth it for performance isn't currently known.
There's also a fair bit of agreement that we're not really happy with the cost of 1U rackmount boxes compared to commodity systems when it comes to the page builders (Apaches). There's a fair chance that we would try the commodity supercomputer approach once we switch from renting racks to renting a room, which is likely to happen once we have filled the second rack. At that point the prices for a room and a rack are about the same and the room is more flexible.
The Opterons are used in part because 64 bit is mandatory to use more than 2GB of RAM for MySQL's InnoDB buffers. So, all database servers are likely to be dual Opteron until we have some pressing reason to switch. I don't recall anyone having a different view on this (though having written it, I'm probably about to find out that someone does disagree and I missed it or forgot it:)).
When it comes to donated equipment, we'd simply accept whatever was offered and raise the priority on getting the load balancing sorted out if necessary. There's work on that proceeding, though at a slow pace since it's not currently urgent.
It's rare for me to read the lists. I just happened to see your question after my mail rules broke and I was sorting by hand. BCCs are usually an unreliable way to attract my attention - the rules treat them like any other list post and file them in a place I'm unlikely to read. Please send a non-BCC copy to me if you really do need a my comments on something.