The 4870 will feature 512MB of GDDR3 memory, with a 1Gb version scheduled for release later this month. The 4850 also features 512MB of memory, but AMD have used GDDR2.
Sorry to point this out (unless i'm missing something) but isn't that kind of old news or just plain wrong in the case of the 4850? Do you mean that the 4650 will have 512MB GDDR2 and the 4670 will have 512MB-1GB GDDR3?
Sorry if I am being an idiot