Hypermemory is not actualy memory... it's "borrowed" memory from the system ram. Just because the GPU has 512MB doesn't mean it won't use hypermemory as well; GPU's have done this ever since AGP was around.
I don't have an ATI card to find out exactly what panel you are looking at, but my guess would be it is simply stating how much hypermemory is "reserved" by the card. As Rob said GPU-Z will tell you pretty quick how much real memory it has.
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