Thanks guys,
The bit stream is indeed much larger than available RAM.
Found a benchmark here : www.roland-riegel.de - sd-reader: MMC/SD/SDHC card library: Benchmarks of 195kbyte/sec that is not so far from the 256kb/sec i need.
Another one here : http://arduino.cc/forum/index.php/topic,118529.0.html states "256000 bytes per second" in RAW mode.
Sounds pretty on the limit.... but may be possible ?
Maybe it is possible to write in 4bit mode ? Do you think it will be any faster ? is there any ready library for this ?