With regards to total power there is no problem. Where I see a possible problem is in the overloading of the individual rails from the PSUs. Since I can only use 1 PSU's ATX rail to power the motherboard, I am only working with 20A from the ATX connector and possibly 20A from the CPU connector.

The 6990 uses 75W (from MB) + 2x 150W (8 pin PCI-E) while operating at 830MHz for a single card (375W total). My question is if the card is overclocked and uses more power, will the power usage grow proportionally from each source, or will the PCI-E connectors compensate the difference required, leaving the MB slot draw at 75W.

Thoughts?