When you do that you run into other problems:
A) I don't know what size your case is, so I don't know what mobo will fit.
B) Prefab computers usually have the cheapest parts available IE: The mobo, power supply, and RAM. RAM should work, don't know what the power of the power supply is, it might not be enough to power the upgrades.
C) Should have just listened to everyone's advice and built your own in the first place. Now you see why everyone offered that advice....
Regardless just adding half of my build will give you 1000% better gaming....based on the GPU alone.
UserBenchmark: AMD R9 280 vs Radeon HD 8470D