Well... did redo the speed test using 319MB size AVI file copied from computer to N800. It took 740 seconds... (that is over 12 minutes...). By reading back the same file to PC took only 30 seconds! Thus the read speed is over 10MB/sec, which is over 20x faster than write speed! That is with "regular" 4GB SD card, not SDHC card. (I do have the SDHC kernel installed which seem to be faster than the stock kernel, even with non-SDHC cards.)

At any rate even Class 2 card would be many times faster to write than my current 4GB card. (Read speed might be slightly slower?)

Based on your recommendation I'll propably just get a Class 2 card.