The thing is, if DF
requires more than 4 GB of memory, it will be practically unusable for a lot of people with machines without a ton of memory. Of course, it's definitely a good thing to make it not crash if it does hit that number, for people who want (and are capable of running) huge worlds, maps, etc., but it's not really reasonable for DF to require 4GB+ in a typical world (unless the average machine's memory capacity improves dramatically, which would take a while).
There are ways to reduce memory usage, for what it's worth, even with those sorts of new features. For instance, only certain chunks of the world are kept in memory at one time - the rest (if they need to be saved) are saved to disk. The usefulness of this tactic varies depending on what sort of information is involved, though - I'm not sure how it would apply to the individual-version-of-events feature (although that would be a really interesting feature, by the way).
A 32-bit OS cannot run 64-bit programs (at least not without a much slower emulation layer).
Didn't say they could.
Oh, did you mean most computers can run 64-bit OS's and programs? That's not entirely true - there are some computers (albeit less common) that still have 32-bit CPUs, which can't run 64-bit code without emulation. Whether it's worth dropping support for them or not is up to Toady (I personally wouldn't use 64-bit support as a reason to drop 32-bit support, but it depends on how much work it would be to maintain, I guess).
Yeah, worldgen can take up more memory than normal gameplay (or less, depending on what you're doing in-game, but normally more).