Mepis is built for 32-bit x86, so on the AMD 64 bit processors, it runs as a 32-bit system.  AMD made the right choice here by choosing to make their 64-bit system completely compatible with 32-bit x86 software, so systems that assume the AMD is a plain Pentium work just fine.

On recompilation:
In order to have your system actually work as a 64-bit build, you actually have to recompile everything, kernel, all of the GNU tools, your favorite applications, etc... for 64-bit, and a lot of apps don't compile this way yet.  For apps you leave as 32-bit binaries, you need to set up one of several supporting regions for them to run with that contain the 32-bit system libraries, etc...  Some builds go full 64-bit and set up a chroot "jail" where 32-bit apps run, others set up a kind of library sharing scheme (which has it's own problems at times).  I'd recommend picking up a pre-built 64-bit distro, since just recompiling the kernel for 64-bit will, usually, break just about everything on the system (I had this happen with a Mepis install a while back).

good luck.

==Joseph++

Eric "Shubes" wrote:
Craig Brooksby wrote:

Here's a dumb question for you hard core Linux types:  I am a "Desktop
Joe" running Simply Mepis on an Athlon 64 box I just built.  I assume
that Mepis treats it as a generic X86 processor.

I don't know, but that would be my guess too.

So: do I need to /
should I recompile from source, in order to gain what -- performance? stability?  I have looked, but haven't found resources relating
specifically to Linux and Athlon 64 so perhaps it's a non issue.  Any
advice will be appreciated.

Thanks --
(the other) Craig

Recompiling the kernel source isn't trivial (unless of course you've done it once or twice). To KISS, I'd try a distro that is precompiled for the AMD64, such as Fedora Core 3 at http://fedora.redhat.com/download/