An AMD FX 6300 would do just fine or something similar from intel. Also for the GPU, I would suggest you GTX 960. As for the ram, as long as it is from a reputable manufacturer it should be fine. If you mix/match ram sticks, you might have some problems. For the ram amount, 8GB seems about the minimum these days for a gaming PC. There are bunch of 2x4GB sets of DDR3 that are decently priced. Hard Drive - If you got one already that you can use - just use that. If not, well, they're not overly expensive, that is if you don't want to go for an SSD.
Most importantly, do not cheap out on a PSU. That's the worst thing you can do, because if PSU dies, you do not know what will happen next. Read PSU reviews, buy psu from a reputable manufacturer and it should be ok.
As for the motherboard, it is all about what you want from it. You can get them for cheap, containing only bare minimum or get some other features, like USB 3.0/3.1 (Type-C), SATA 6Gbps, more expansion slots, better on-board audio etc.