R580 features a robust memory controller capable of utilizing several different types of memory, including GDDR4 which was not even available when the Radeon X1900 was first announced. The new GDDR4 variants of R580-based Radeons are now called Radeon X1950. Radeon X1950 will retain all of the features of the Radeon X1900, and really only have the added benefit of a new cooler, GDDR4 memory and different frequency clocks.
Here comes GDDR4! I have to say, the best part of the X1950 may very well be the reference cooler. That aside, both cards will include 512MB of memory and higher clocks that previous offerings.