Currently in New Zealand looking to buy a second gaming computer for myself. Will mainly be used for playing Diablo 3 and doing photoshop work.

The parts i've found so far are these:
http://www.pbtech.co.nz/index.php?z=p&p=KITPB1700&name=AMD-FX-Series-FX8320-Eight-core-3.5Ghz--Gigabyte-9

But i'm having trouble picking a graphics card:
http://www.pbtech.co.nz/index.php?z=p&p=VGASAP8270&name=Sapphire-R9-270X-2G-GDDR5-PCI-E-Video-card--Dual-D
or
something else, I'm new to this but why is a 270 graphics card more expensive then other ones like 550 etc? Can someone explain


Pretty much the idea of this is to buy these similar parts for the cheapest. New zealand being VERY expensive with computer hardware, i would prefer ordering parts
Can someone help me out?