The HD2900XTX is rumored to have GDDR4 some new samsung chips that are clocked at 2,400 - 2,600 stock and will run upwards of 3000Mhz... About 10-12 Gigapixels per second. And two flavors will be available 512MB & 1GIG. About 10 Gigapixels per second.
The cores will run anywhere from 880MHz to 950Mhz in stock and overclocked retail versions. Knowing AMD, with the correct cooling and adequate power, I"m sure we'll be running this cards at Ghz speeds in no time.