Windows 3.1 should run well. I have installed XP on a PIII laptop with 512MB of RAM, it RAN OK, with the base setup. You won't be doing much video watching on that thing though. Find a used P4, they sell for 60-100$ and will run Windows XP much better. Look for a 2.8 gig or faster system, and a 333 or a 400 mhz bus.
Throwing money at an old system is hardly worth it.
Any version of Windows below 98 is very dated and you cannot get any support from Microsoft.
Even Windows 98 is fairly limited by today's standards and that also gets little or no support.
I agree that 512mb is almost a minimum these days, but I would not bother even getting more ram for a very old pc.
Even 5 years is old by modern pc standards and components for old machine are hard to find.
I would save up for a good second hand machine running xp or Window 7
By the way when asking questions it is helpfull if you give more spec,about your PC.
Best of luck