It depends. I recently drilled my mother board holes and I used an M3 tap but when i went to check the placement with some standoffs from another case, they were 6-32. If the threads on your standoffs are the same as a case screw then they're probably M3 but if they are more coarse than that, they are probably 6-32.
it all depends on where the case was made, americans use there version of imperail measurments. If you pick up a Lain li or a non american case then you would have a meteric threads. M3 is the chap to use!
They're all made in China, whatever the brand. But if you buy standoffs, even in America chances are they're M3 like these.
The male end of the stand-off is always UNC #6-32. The female thread on the stand-off can be #6-32 or M3. I tend to prefer #6-32 because there's a wider variety of screws available, and it's marginally easier to tap. Make sure you purchase a tap rated for tapping the hardness of material you are working with.
