Toms just wrote an article about the limitations(or lack of) when running video cards on boards such as the p55 (will do 8x8) and x58 (will do 16x16). I know you are going AMD, but it's the limitation of 8x8 that you are looking at. The performance difference was rather minimal (I think around 5-10% was the biggest difference) unless you were using top tier cards, which the ones you've selected are not.
Ultimately, it would be better to have 16x16 support, but I would not say its worth more than $50 in an upgraded motherboard. For the extra money, you could get better graphics cards that would negate any benefit you'd get from the 16x16 speed.