SO VERY MUCH OUT OF DATE
Ask about 64-bit stuff in #sourcemage on irc.freenode.net. That's your best bet for the latest information...
SourceMage Status on AMD64 Machines
Thanks to Benoit's work, we now have 0.9.4 test ISO for x86_64 platform. Several reports have been made that the CD works.
You can choose to keep both 32-bit and 64-bit libraries, as is done on the ISO; this is called "multilib" configuration. Or you can surgically alter the system after installation and make it pure 64-bit.
Multilib: Keeping 32 Bits
This option allows you to run 32-bit programs in 64-bit system. However, 64-bit program cannot link to a 32-bit one, neither dynamically nor statically. Hence, you wouldn't be able to use say 32-bit plug-in in 64-bit Firefox.
Personal plug: I actually suggest we drop 32-bit support on 64-bit systems, it's just too big a feature for too little gain. I say, if somebody needs to run 32-bit applications, use 32-bit ISO.
Pure 64 Bits: Don't Look Back
This option will make your system truly 64-bit, without the possibility of compiling or running 32-bit software. You gain performance (not always), but you also become vulnerable to problems some spells face when compiling for 64-bit execution. Now that both Intel and AMD are pushing 64-bit processors, more and more software will be ported to 64 bits, hence going forward this may be a better option.
How to do it? After installation is over, boot the system first and make sure you can login. Then boot from the CD again (or from something like Knoppix), go into "repair mode" if you're using the SourceMage installation CD, and follow these steps:
Mount your root partition;
Move files from /lib64 to /lib;
Symlink /lib64 to /lib;
Move files from /usr/lib64 to /usr/lib;
Symlink /usr/lib64 to /usr/lib;
Remove references to /lib64 and /usr/lib64 from /etc/ld.so.conf;
Run ldconfig -r <your root partition>; you can also include -v to watch it;
Reboot into your system and start using it.
Known issues with test ISO
Multilib headers are used instead of regular headers. That means some spells might not like them. Most have been fixed, and since multilib isn't needed, you can use gaze alien to remove /lib/include/* files after rebuilding glibc, but be careful and this isn't really needed to get a usable system.
After upgrading the grimoire, cast gcc first, then cast g++. That's the best way to update to the new split-compiler layout. Casting g++ first will lead to an unusable compiler and you'll have to restore the specs file from somewhere else (drop in IRC to grab a copy)
Make sure to compile ncurses with utf-8 support on. That was accidentally enabled on the ISO, and it's complicated to go backwards to a non-utf-8 supported ncurses without using a statically compiled bash (found at /usr/bin/bash.static). If you don't do this correctly, grab an ncurses and readline cache from somebody in IRC and resture them on your install, or use the static bash if that works for you. (Edit: bash.static is broken on x86_64 currently, so caches are the best option right now)
Since x86_64-0.9.4-test3 iso
(NEW) All the above issues should be fixed at least in stable-rc grimoire, we just need to release the next test iso. The ncurses issue is still in stable grimoire.