Upgrade Advice: Graphics Card and RAM

Hello! I was wondering if someone could recommend a graphics card and what type of RAM I should get to upgrade my current system. It's my understanding that the graphics card will depend on the power supply. I would like to upgrade to play games like Overwatch, Fallout 4, ARK, etc...

Here are my system specs: Brand iBUYPOWER Computer

Model Gamer Power WA563R7

Form Factor Tower Only

Weight Approx: 30 lbs

Chassis ARC 647

Chassis Dimensions 19" x 7.5" x 19.5"

Processor AMD FX-6300 Vishera (3.5GHz/4.1GHz Turbo, 8MB L3 Cache)

Processor Cooling Fan

Processor Socket AM3+

Motherboard AMD 760G

Graphics Card AMD Radeon R7-250

Video Memory 2GB

Power Supply 400 Watt - ATX

Memory (RAM) 8GB DDR3-1600MHz SDRAM

Memory Slots Total (Free) 2 - 1.5V DDR3 DIMM Slots

Max. Capacity of System Memory: 16GB*

Hard Drive 1TB - SATA III - 7200RPM

Optical Drive 24X Dual Format/Double Layer DVD±R/RW + CD-R/RW Drive

Networking (LAN) GbE LAN chip (10/100/1000 Mb/s)

Audio 7.1 Channel High Definition Audio

Software Microsoft Windows 10 Home

Front Ports 2 x USB 3.0 Port 1 x Audio/Mic Port

Rear Panel I/O Ports 1 x PS/2 Keyboard/Mouse Port 1 x D-Sub Port 1 x DVI-D Port 1 x HDMI Port 4 x USB 2.0 Ports 2 x USB 3.0 Ports 1 x RJ-45 LAN Port w/ LED 3 x Audio Jacks: Line in/Line Out/Mic

Expansion Bays Total (Free) 3 (2) - External 5.25" Drive Bays 3 (2) - Internal 3.50" Drive Bays 3 (3) - Expansion Slots

Expansion Slots Total (Free) 1 (0) - PCI-E x16 Slot (Running x16) 1 (0) - PCI-E x1 Slot 1 (1) - PCI Slot Storage Controller (Total) 6 (4) - SATA2 3.0 Gb/s Connectors

Thanks in advance!!! :D