I would upgrade the graphics before most things. The A8 should be fine, yet you are limited to a 300W power supply in the HP case assumedly, since HP usually uses 300W's in their towers.
That being the case, your best bet would be the 7750. Like one of these:http://www.newegg.com/Product/Product.aspx?Item=N82E16814102969
You won't be playing at max settings, but if you want to, you are going to need a new power supply as well as a more powerful graphics card, which is going to set you back at least $200. The 7750 is the most powerful upgrade you can get with your current power supply, and will play all games, not at the highest settings, but will play them.