I'm curious to see what the 64-bit memory-tax is for various languages/runtimes.

I took a quick glance at the Language Shootout to see if I could find any patterns, but at least for the few I checked, it's all over the place. For example, on the "n-body" problem, both Ruby and SBCL grow 31% on x86-64. But Ruby on "pidigits" grows only 2%, while SBCL on that problem adds a whopping 111%. On "spectral-norm", Ruby grows 61%, while SBCL grows 31%.

So this appears to be a place where moving to 64-bit is completely unknown, and you have to test for your specific application. It might double your memory usage with no change in speed, or (if you're running pidigits.rb) it might double your speed with almost no change in memory usage.