are u alright mate ?! , u are adding 16+4 and 8+8 , and u say 16x4x wins ? please read some reviews , the best dual board is a board that supports 2 cards in full 16x mode(both @ 16x) ,
another option is a motherboard that supports both graphic @ 8x mode(8x8x)
the last one is 16x 4x , which performs alot weaker than 16x16x and 8x8x8