I found some information about my question.
SWTOR uses DX9 and AMD Dual Graphics (the asymmetrical CrossFire implementation used by AMD APUs when paired with discrete GPUs) only works with DX10 or higher.
Pretty much any GPU equivalent to AMD 5570 or greater (including A8-3870K) can run SWTOR on low settings@1280x720. For 1920x1080 more GPU power is recommended.
Discrete GPUs in the performance range of HD5570 will be faster than A8-3870K due to their onboard DDR5 RAM.
If you play SWTOR in areas heavily populated by other players (like PVP or Operations) more CPU is needed than if you play solo PVE.