you mean TAPS? drill bits drill holes, taps thread the holes to make machine screws go in the holes you drilled.
6-32 is the universal computer thread tap/screw. next size down is 4-40 (cdrom/hard drive thread size)
I would get the following sizes taps:
8-32
6-32
4-40
Can't hurt to have these sizes laying around. If you go to any huge hardware store you could buy a tap & die set, which will usually come with tap & die as small as 6-32, but not 4-40.
Like everything, I get it from McMaster Carr.