all my answers is base that you plan crossfire whit your 2 rx 480

1- for gaming GPU first, then CPU.

2- cost efficient, well depend of what you mine, radeon are mostly ethash mining. Personnaly i find a 1070 best for a mining-gaming rig (event a 1060 6gb is pretty good-all around)

3- sry dont know what market is at malaysia, but i can say you, a i5-6500, a board whit crossfire try aim for know manufacturer (msi, asus), for PSU aim for a gold (bronze is still ok if not budget). use a small SSD around 240gig and a big normal one (better cost effective), if you can 8gig is good but aim for 16gig.

4- need lot more info, like aim algo, crossfire? single gpu mining 24/7? budget? custom bios or default bios gpu? mining for profit or paid cost of hardware?