Your GPU is just pure shit.
You need 4 more GB of RAM.
You need better PSU.
For GPU I would recommend MSI R9 380 GAMING (around 200€)
Get same ram that you already have(4GB) (20-25€)
For PSU I recommend Corsair CX750 Builder Series ATX 80 (60~€)
If you want to spend all 400€ than go for R9 390 instead of 380.