Is it possible (and safe/reliable/non-damaging) to use a 20-pin ATX PSU with a 24-pin ATX motherboard?

"I have no exact information to provide you here, but my gut feel would be that you should only consider trying this if you have a relatively low-power system on the motherboard. Something with not too much memory, ideally no more than 1 hard-disk, and probably not too high-end a dual-core processor... if you have any kind of modern graphics card installed I would probably not even consider going there.

Note that what the 24-pin supply does, is add one additional supply line of +3.3V, +5V and +12V each. If you use the adapter you suggest it will copy one or more of the existing lines to supply these pins... which means that they need to share the additional load.

From having a look at the Wikipedia page you reference, it looks like the standard 20 pins have a decent number of +5V and +3.3V lines already, but there is only one +12V line, which gets duplicated by the extra 4 pins. I do not know offhand what hardware uses the +12V, but that would be my main stability concern when using the splitter attachment."

"It is not recommended as exceeding the current draw available will cause the PSU to either lower voltages (which could damage components) or simply shutdown in overload. My recommendation is to buy a new PSU. Really, they're not expensive.

However, if the motherboard has a Molex socket, then use that as well as the 20-pin plug and you'll be fine. There also used to be adapters from a Molex socket to the 4-pin extension, which would also be fine."