Well a 300W power supply won't give you much room for upgrade. You'll want to spend like $50 or so on it. Then you'll have to reconnect all the cables and it could be a pain if you don't know what you're doing.
As for a keyboard, look into mechanical keyboards. They're really nice to have.
As for a mouse, you don't need to spend a lot. A mouse with adjustable DPI is usually pretty nice to have.
This is what I have.The GTX 760 is a nice upgrade, but your CPU may or may not bottleneck. I don't know a whole lot on the A8 series.
But you seriously will need a new Power Supply. Almost any 550 watt or above works.
