Definitely a new PSU and of course throw in some capable GPU's, I'd start with 1060 's while you are starting out. It's a myth that you need gaming or mining motherboards

I use a crappy matx motherboard with one PCIe x16 and two PCIex1 and a cheap PCIe expansion card I easily run 6 cards. You don't need top ram or processor either mining is GPU intensive not CPU, RAM or disk intensive.