Oh right so you don't have the PC? Errm try em then. If they're longer stick a washer in, don't tighten them all the way without a washer, cause otherwise it'll crack your board.
Oh right ok good good. Errm for that, I can't say. If you've got one of those measuring things (don't know the proper name), you could measure & compare them both. Or you could just try it on the hole and see.
Yep, PC screws are either #6-32 (most of them) or M3 (the smaller ones used for optical drive mounting, motherboard etc.) Most grip screws are, conveniently, #6-32.