I'm going to build a mixing computer for live events, using three or four BM intensity pro cards I already have lying around. Inputs come in from cameras via HDMI, output has to go to a 1080p projector via HDMI. I wonder which option will result in less video latency: should I connect the projector to the secondary output of the graphics card (GTX 670), or should I dedicate one of the intensity cards for HDMI out?

I have yet to buy cpu, ram & mainboard - can you recommend a suitable combination? Especially a mainboard with at least 4 pci-e x1 slots plus 1 pci-e x16 slot (double height, to fit the GTX 670) usable at the same time? I don't need recording capabilities, and i'm only switching live feeds, no video files - would a moderate intel i5 or even i3 be sufficient?

Thanks for your advice!