It may depend on which way around you installed the Ram.
The highest Ram should go in slot one then the lowest Ram in the next. So in your case the 1Gb sticks should be in slots 1 & 2 with the 512 in slots 3 & 4. Some board do take a lot of Ram, but remember it all may not show up but it is still there. One of the reasons for this is that if one stick fails the other will take over.
I cannot see an issue with the two different manufatures sticks as long as they are of the same type.