The catch is, the addon GPUs can not be a higher model than the onboard one. So I am looking at Radeon HD 6670 cards:
I wouldn't sacrifice performance for the sake of utilizing the onboard video. Just get a pair of whatever video cards you want most and forget about it. It's not going to provide any substantial improvement and you're only intentional crippling yourself trying to use it.
It's looking good, though I would have gone with a modular power supply. All the extra cables drive me a bit mad. Have you booted it up yet?