Yes, and that's all nice. But what are the advantages? I, and probably most other non-technically minded folk here, really don't care what's not possible. Basically you're saying Toady has been fooled into wasting time making a 64 bit Dwarf Fortress which does nothing and for some reason wants us all to get excited about the nothingness. Great.
Ever since DF2012, (which introduced the expanded worldgen populations,) it has been impossible to run worldgen for the full 1050 years on the large or even medium world sizes because it would overflow 2gb in memory. Making it so his game can run as he intended for it to run is the only reason he needs to make the investment in time to create a 64-bit version of the game.
Things like being able to better take advantage of a 64-bit CPU architecture are nice, but far less valuable than just going back and optimizing the base code, which would likely take far less effort on Toady's part, since he openly admits he has serious room for optimizations, yet. There wouldn't be reason to switch to 64-bit for that, alone.
Regardless, I didn't mention building your computer around the upcoming change to 64-bit because there isn't any reason it should change your purchasing strategy. You still want any reasonably modern middle- or top-of-the-line CPU with the biggest L4 cache you can buy, and DDR3 RAM (DDR4 isn't quite there yet with speed, as far as I've read, and costs far more, check back in a few years) with the highest memory clock and lowest CAS latency you can buy.
Hypothetically, yes, 64-bit means you'd have a reason to buy more than 4 GB of RAM, but seriously, who's buying an i5 processor, and only slotting 4 GB of RAM in next to it in 2016? It ultimately means nothing to the type of computer you'd want to buy, since, unless you're literally buying a computer ONLY to run 32-bit DF. In most cases, you don't even have the option to buy something that doesn't run just as well in 64-bit architecture without looking through garbage dumps or cobweb-strewn backs of warehouses.