not knowing much other than your screen res...(bc the link you provided does not work) I would suggest getting a 6850 or at least a 5770, starcraft and wow are more cpu demanding so knowing your full system specs is important, your expectations may be set a bit too high if you have a weak cpu
the max gpu you could run from that 380w PSU would be a ATI6850 or nvidia gtx460. However your performance will be severely limited by your slow old CPU. You are better off upgrading cpu/motherboard first, you will see improvements from that alone. The fastest card I would put in would be a GT240, but you wont see much improvement without upgrading CPU.. NVIDIA cards work better with the games you mentioned.