EMT64 Xeons -- we got two, but the motherboards from SuperMicro were from a faulty manufacturing run and have to be recalled... they passed burn-in, but they'll only boot once before the CMOS battery dies. Unfortunately we can't get replacements because Intel has apparently told SuperMicro and all other vendors that sell Intel's Lindenhurst 800Mhz FSB EMT64 chipset to stop, because Intel's found bugs in the chipset. This might be confidential, but I didn't sign no NDAs. I heard it from a little birdy who heard it from another birdy, and so on, and the birdy telephone game just results in a lot of confusing chirping, so I don't guarantee anything I say here is the truth... just what I decoded.
AMD Opterons -- incredibly spotty availability. some weeks only the high-end ones are available, and only a few, and some weeks only the low-end ones are available, and only a few. plus the motherboards are flaky compared to the robust server ones available for Intel chips.
Itaniums -- see above w/ spotty availability. Apparently these aren't stocked well in the US? Then where are they stocked well?! Plus, who uses Itaniums? But at least it's an official Debian-supported architecture. (amd64 will be soon....)
SPARC -- Solaris OS; Sun pricing. I'd like to run Linux, just for consistency. Solaris 10 did impress me, though, but I don't have time to learn it on a production system we needed 3 weeks ago. I want to learn Solaris 10 in isolation without stress.
G5 Xserve (update) -- only 8GB of memory? oh, and pricey.
In conclusion: Bleh.
We waited quite some time for the EMT64 Xeons, but now we can't even get one to work. I had assumed we could always go AMD (and betray the family that fed me) but that's not even looking like an option.
BTW, the reason we need 64-bit processors is so InnoDB can use tens of GB, and not just ~2GB.... 32-bit address space is just so tight.
But much love to Silicon Mechanics and SuperMicro for working so hard to get this all working for us. A fix might be happening afterall. We'll see. I guess this is what you get on the bleeding edge.