I just want to upgrade my rig just enough to play games on medium (games like diablo 3, terra, starcraft 2)

CPU ~ AMD phenom x6 1045t 2.7 ghz
video card ~ ATI Readon 6450
RAM ~ 8gb DDR3
PSU ~ 300w

My budget: $150-$200
At first i was thinking of just upgrading my video card but I'm probably gonna need to get a new power supply as well.

Some cards I was looking at were the radeon 6770 and the gtx 550