Well, keep in mind that those benchmarks were using a 5870 - more powerful than all but one of NVIDIA's cards, and also extremely hard to find as well.
And you are correct, AM2+ motherboards only support DDR2, and AM3 ones only support DDR3, that's the only difference between the two. But if there is a price difference between an AM2+ and AM3 model of the same motherboard, it's probably nothing more than 5$, I'd imagine.