If it were me, I would first get a good psu > motherboard > cpu > video card.
For you, I think you should get a new Psu + Video card at the same time since you like to game.
Here's the problem that I see when looking at your system.
If you get a newer video card, the cpu might hold you back and your cpu can't be overclocked very will with your motherboard. Plus a 400 watt psu might be too low to handle the newer video card.
If you get a new psu it doesn't really help your games much.
If you get a new motherboard, you can maybe overclock your cpu better which should help games abit.