HD 2900XT

Core Clock:...............743 MHz
Shader Clock:............743 MHz
Memory Clock:...........825 MHz (1650 DDR)
Memory Bandwidth:....105.6 GB/sec
Shader Operations:.....237760 Operations/sec
Pixel Fill Rate:.............11888 MPixels/sec
Texture Fill Rate:.........11888 MTexels/sec

HD 3800

Core Clock:................825 MHz
Shader Clock :............825 MHz
Memory Clock.............1200 MHz (2400DDR)
Memory Bandwidth:.....76.8 GB/sec
Shader Operations:......264000 Operation/sec
Pixel Fill Rate:.............13200 MPixels/sec
Texture Fill Rate:.........13200 MTexels/sec

not smaller la, the core die based on 55nm, so it smaller compare current r600 which based on 80nm and r630 which based on 65nm. it 256bit
smaller die size produce less heat