Depends on your setup, if you have a power hungry GPU then you need to prepare more headroom for it. For the current GTX1070/1080, a decent 400 - 450W is already enough, if you are planning to stick with AMD then get a 500/550W just to be safe, their power consumption never amuse me so extra headroom is always recommended.

Difference in terms of overclocking? Do you mean CPU overclocking? If you are using Intel CPU then overclocking doesn't take a lot of power from the wall, just a small increase. The 80+ rating is mainly for efficiency, it does save you some money in your electricity bill, but that will take quite a long time before you can benefit from it (as in the money paid for a 80+ Gold PSU vs the money saved from the bills).

GPU upgrade, well it depends on your budget TBH, even if you decide to stick with 1080p in the next couple of years, you still can invest in a "1440p" card, i.e GTX1070/1080, this gives you the opportunity to run all the games at ultra settings with decent frame rate without compromise. But don't forget you have to prepare at least 3k for the monitor too, 1440p 144Hz is pricey in Malaysia, so GPU and monitor upgrade would cost you nearly 5k+.