Just experimentally, I have just run beta5 on my HP G62 2.13GHz i3 laptop, using 1GB ram with an 80GB HDD, using Web Positive to access the Haiku forums, & BBC Weather website.
It was a little bit slow, & was swapping a bit, but for such minimal resources, amazing!
So now, I’ll upgrade it to 2GB ram…. 
Needless to say it was faster….
Now fitted with 3GB ram, seems to run perfectly smoothly, & still running from that old 80GB HDD. 
Slowest hardware I ran Haiku was on an Athlon 900 MHz, 256 MB of SDR RAM. But that’s a bit cheating, because it was on 2005, when Haiku only booted into “consoled” (basically just a text console running bash), and little more than that 
(I was still able to use it to listen to FM Radio with a driver I wrote for my TV/FM card).
Did some test a couple of years ago on a Sempron 3600+, 1 GB of RAM, went smoothly (after a BIOS update that added missing widescreen resolutions).
I still have a couple of Atom N450/N455 with 1 and 2 GB of RAM, where Haiku doesn’t seems to mind very much the lack of CPU power (if you don’t try to compile more than a few lines of C++, and stay away of graphical web-browsers, that’s it 
).
I run Haiku on an Asus EeePC 701 with 512M of RAM and 4G of storage. Web Positive is obviously not a realistic option (though it starts in a pinch), but lesser apps generally run. The tiny screen is a bigger obstacle to usability than system resources, though the 900MHz Atom makes it very hard to build much at all from source.
I run Haiku on an unmodified Dell Optiplex 7010 which is a CSM machine. I can run Haiku booting from Legacy BIOS off-the-shelf, but with UEFI only with failsafe videomode.
I did run Haiku on a Pentium 4 @ 1.8 GHz and 1G of Ram (It still has the 20gb hard disk which was in there originally) and used it to do a few haikuports recipes (iirc in 2022). I remember it as completely usable, even with github being open in webpositive. Though compiling 7kaa was a bit time-consuming (~1hr on a P4 compared to a few minutes on a Core 2 Duo iirc). For some reason I tried compiling inkscape on it, but gcc eventually produced an internal compiler error after 1 1/2 days xD. The thing also transformed into a space heater during that time.
I also briefly tried it on a HP T5710 thinclient with 256mb of Ram and it booted, though I only tried the live environment. The 256mb of ide flash is just to little to install any modern os really and I doubt that you could use that system with 256mb of ram productively.