The 2.6GHz Celeron would easily walk over your E7400 at stock. You'd have to overclock it to ~3.3GHz to get on even ground performance wise.
Occasionally driver issues causing games to work better using only a single GPU, etc. Microstutter as well (although the issue has been much...
Crossfire on a single PCB is still crossfire, and comes with any and all potential hassles that might entail.
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