Just installed Windows 10 Pro (x64) using 2gb of memory!  Yes, that lame-o amount of memory worked fine.  I did not say it worked well!  Performance is very lacking after installing a few third party programs, especially AVG Internet Security suite and IOBit Advanced SystemCare.  With only the operating system, 2gb of memory actually performed quiet quickly.  I was impressed.  The system was just an old D945GCCR mainboard with Intel E4500 processor.  Disk drive was also old SATA-II Western Digital Blue 250GB.

All-in-All, this system could be used on a desktop for basic (very basic) use (word processing, spreadsheets, etc) but hooking up to Internet and being more productive.... needs more memory.

Bottom line, it is worth it (in my humble opinon) to do the free Windows 10 upgrade from Windows 7.  If your hardware is older,  just upgrade the hardware (add more memory, maybe faster processor, etc.) later when you can.   Can't beat free, Microsoft ends the free upgrade soon.