The x86_64 architecture is a multilib architecture. To be standards compliant, a distro should support both the 32-bit and the 64-bit ABI side by side. IMHO, stay away from "pure 64-bit" distros. You'll be using wrappers for things you really don't need to wrap and, again IMHO, it unnecessarily complicates things. So, I would advise looking for a multilib distro.
IMH(and biased)O, Cross Linux from Scratch is the simplest, most straight forward multilib distro for the x86_64 target that I've used. The CBLFS Wiki
has build instructions for about 1000 packages including a whole section for Science/Engineering/Mathematics packages. There are also many audio/video/multimedia packages represented.
If you're not into rolling your own OS, Gentoo is excellent. I used Gentoo before CFLS and keep a minimal install around as a backup/recovery that I can boot into if I hose something on my CLFS install. Gentoo has a very large repository of packages including math/science and video apps.
I have recently been playing around with Slamd64. Version 12.0 has multilib support. The layout is a little more involved than CLFS or Gentoo and I have had problems building some 32-bit packages, 64-bit packages build fine. However, I haven't had any problems yet installing 32-bit binaries using slack packages so from a runtime perspective Slamd64 looks good.
I've had some experience with Ubuntu's x86_64 offering because both my kids used to use it. I won't say anything more than that and let you draw your own conclusions from my silence.
If you're doing heavy-duty number crunching, you will notice a difference using 64-bit applications. I simulate the behavior of complex, repairable systems over 10-40 year periods and varying maintenance policies. One simulation used take about 40-hours on a 32-bit machine. When I switched to a 64-bit machine, simulation time was reduced to about 26-hours.
As far as a motherboard, I have been pleased with Gigabyte boards. I have built four x86_64 machines in the past two years and have used Gigabyte mobos in all four. Every one worked superbly with Linux, everything worked "out of the box."