Yeah dude, ASUS is probably the best motherboard manufacturer, if not, the second best. Either way, it's up there so i wouldn't go needlessly bashing the manufacturer about your problems. I suggest doing some of your own tests like taking out the 2 old memory modules and just using the new ones to see if they work alone. Then try using just one of the new ones along with the old ones, see if that configuration works, then swap out the new RAM module for the other new module and test it the same way. That way it is easier to single out if one of the modules isn't working properly.
Of course i am not also saying that you should entirely rule out the motherboard because you could be right. There may be a problem with the memory controller. Also, it is not normally recommended that you buy 4X512MB because it is said to put added stress on the mem controller (but not that it really matters to you at this point i suppose).