XP64 will give you an OS with a lighter footprint than 7 or 10, but with simple access to your entire 6 GiB of RAM. My only caveat would be about driver availability, but you mentioned that wasn't a problem with your hardware.
I'd consider XP32 best for compatibility with pre-XP software, though.. especially 16-bit era software.
Is dual-booting an option for you? If so, it might be worth your time to install both XP and a more modern OS. Windows 7 is really all you need for modern browsers at this time, and can be customized from instructions on this forum to reduce the number of running background services and graphical effects. Both should reduce CPU/GPU usage and help battery life.
I wouldn't want to run 8 or 10 on that hardware unless I had specific software that I wanted to run that needs a more recent OS on bare metal.