Both GPU and CPU are not the strongest.
Suggest you look for published frame rates for the fx-4100 and hd5450 and see if you are worse or roughly the same. I suspect that you are already getting all you can from the home theater (not gaming) video card and the not-real-good for gaming CPU.
Look at the last pages of these two articles to see ratings of CPU and video cards. Find your cpu and gpu and see what's stronger. Video cards are very easy to replace and will have the most gaming impact. CPUs are harder, especially if you needed a MB swap.http://www.tomshardware.com/reviews/gaming-graphics-card-review,3107-7.html