Experiences with using a PIII and 512MB to run Win8:
Well, it is slow. Not I want to pull my hair out slow, but slow. Example, I click the close button on a window and it takes about 10 seconds to completely close. Now, I don't know if this is actually the Window taking that long to close, or just how long it takes for Windows to play the stupid animation of the Window fading away. Since there are no proper video card drivers, anything with animation is painfully slow.
At first none of the hardware was really detected, not even the NIC. So I had to connect a USB Wireless adapter that had Win7 drivers to connect to the internet. Once I did that Win8 installs every piece of hardware except the video card without me having to do anything. It just detected the internet connection and immediately started searching for drivers. I like that.
Now, a lot of things seem to crash when I try to open them. IE is a no go, it just keeps crashing. Now I don't know if this is because of Win8 not liking the system, or because of some fault with the system. I only have 2 256MB sticks of PC133 available to me, the rest I have are 128MB, so one of the sticks might be bad. Services.msc wouldn't open either. However, system properties and msconfig work fine.
Metro is totally useless, again propably due to no video card drivers causing extreme lag when doing anything. I just disabled it.
Booting and logging onto a usable desktop takes about 4 minutes from a cold boot, not too bad considering this is a machine that was purchased in Jun 2001, so it is 10 years old at this point.