Help me buy a pc for StarCraft 2?

That is not a particularly powerful PC. The dual core A4-4000 APU probably has about the same processing power as a Core i3 running at 2.0GHz. The Radeon HD 7480D graphics core is about as powerful as the Intel HD 4000. Overall, I would say you can get around 35 - 40 FPS at 1366x768 resolution with medium settings or 1600x900 resolution with low settings.
 


Neither of those systems come with an operating system nor a video card good enough for games, and the CPU in them is on the low end. You can put together a low end gaming system for about $500, or you can look for a used one in your area for less. Whoever is selling those as "gaming systems" is flat out lying. They are good for web browsing and movie watching unless you add another $2-300 worth of parts to them.