The 4850 and 4870 will both have the same chip, both the full version so no disabling of parts. Clocks on the 4850 are 625 MHz. and for the 4870 it's 750 as reference speeds. 4850 will have 512MB GDDR3 at just under an effective clock of 2 GHz. (64 GB/s bandwidth) and the 4870 will have 512MB of GDDR5 with an effective clock of 3,6GHz. (115GB/s bandwidth). That's the specs for the reference design cards. There will of course be makers of boards with standard overclocked versions and more RAM, there have already been reports/announcements and sightings of boards with 1GB of RAM for both the 4850 and 4870.
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