A computer is like a big chain, where every link is only as strong as the weakest one. each component depends on other components to perform at it's best.
Cheap RAM will cause a performance decrease over quality RAM. even if it's a reputible brand, they will still have different quality levels within their own lineup. So if it's high performance you're after, then look not only at brand names but also timing.
Motherboards do perform differently, however they have a minimal effect on performance when compared to other components. generally a more expensive board will use better quality parts, have more features, and provide good support.