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Why we don't have 128-bit CPUs
(www.xda-developers.com)
This is a most excellent place for technology news and articles.
You can also toggle it on precompiled binaries with the right tool (or a hex editor if you're insane), which was my main use case. Lots of old games that never got 64-bit releases that benefit from having access to the extra RAM, especially if you're modding them. It's a great way to avoid out of memory crashes.