As for the availability of 64 bit hardware, it's probably more common than you think!
We are a reseller and OEM of computers and servers. Not a big one, but representative I suspect. Virtually every machine, desktop and server alike, that we have sold in the last year is 64-bit. We sell mostly AMD-based workstations, but even the Intel-based machines are 64-bit too. We don't even really mention it to people since we aren't really brave enough to install 64-bit Windows anyway.
I also suspect there is a lot of SME installations on new hardware. Frankly, if you are charging for your time (or paying for someone elses time), it is almost never economic to mess around with used hardware.
I've been running 64-bit Linux on my home and office desktops for going on three years now. Yes, it was painful at first, but it's surprisingly functional and easy now.
I think a x86_64 SME Server would be a great idea. It might even help convince the rest of the world that the move to 64-bit is really worth it.
Though it might be dificult to quantify (benchmark) the advantages of x86_64, if 64-bit SME just came out in a stable form, that would be fantastic.
G