Simply thinking of the concept of quantity, quality, or price takes an account here. If you plan to continue expanding, buy the 1GB. Otherwise, buy the two 512MB and save urself about thirty or so bucks.
Remember, in any situation, you can only attain two of the concepts.
1GB = Quality + Quantity(later expansion)
512MB = Quality + Price(cheaper)
In both cases, quality should be about the same.