New computer help (graphics card)

[Meowingtons]

I bought a Zoostorm Pentium DC E5800 3.2GHz, 4GB RAM, 1TB HDD, DVDRW
Motherboard - ASUS P5G41T-M LX2/GB/SI
300W PSU
8 x USB,
1 x PCIEx16,
1 x PCIEx1,
2 x PCI,
5.1 Channel Audio
DDR3 1333
2 x 2GB DDR3 1333 Sticks in the unit

What I want to do is install a graphics card (dont know much about computers) and I need a better power supply, what would be the best? (for battlefield 3) I was looking at the GeForce GTX 560 Ti'Maximum Graphics Edition' + would this work with my pc and could you suggest a power supply?
Thanks in advance

 

[Meowingtons]

I cant find a picture with the case open + I am waiting for the pc to arrive still. I did some researching and its apparently a std ATX unit case