Reclan Anderson :
I have an AMD A8-6600K, and I want to know if buying, say, a $100 graphics card would be an upgrade.
How good are the graphics on this APU?
The graphics on your APU is low-end performance. Even if you get a 6670D and crossfire with it, you will see some performance but your APU will bottleneck the card because DDR3 vs DDR5 on a dedicated GPU will always outperform. The mid end APUs are the A10k because they come with newer models and some of them R7.
If you really want to increase your performance in gaming I would scrap the APU graphics and just go straight to a dedicated card like R9 270 or higher model ones. At least that way you will have no bottlenecks since A8 is comparable to a FX 4300 cpu.
But for you with an APU there are only 2 options ahead:
1. Either buy a low end dedicated card like 6670D and crossfire that with iGPU and get the increase in performance or,
2. Buy a dedicated card and scrap the iGPU which gives you more performance than crossfire.
Depending on how you want and budget wise, option 2 is optimal.