8/8 is an insignificant drop, it is about 4% on the most powerful cards, the weaker the card the less significant the 'bottleneck' will be.
An i5 750, P55 mobo and Dual channel RAM kit costs significantly less than an i7, X58 and Tripple Channel RAM Kit, and in terms of gaming performance you lose nothing but save enough to get a better graphics card. So, technically for the same price, an i5 750 will give you better gaming performance.
Of course, it seems now there are P55 boards that allow 16/16, though personally I would not spend the money on it
16/16 down to 8/8 is considered nothing to worry about. Only going from 16/16 to 4/4 is there a real loss.
Best board for money in my opinion (for what its worth) is the Asus P7P55D E Pro.