just a quick report - got my new 8GB class10 Duracell SDHC card - and it seems to be working just fine. Some teething problems though (as always) - after first map data copy onto the card all was working fine (unit turned on as fast as possible, was able to switch between maps stored on internal memory and new SD card) but after 3rd or 4th restart it just hang at TT logo, removing SD card seemed to help and unit was turning on, after inserting card again – unit was crashing.
Simple solution after further forum reading - even though card came formatted in FAT32, after "manual" format all problems are gone.
now I got both EU and USA maps available on the card.
very strange... indeed - class declares 'writing to SDHC speed', and reading from it - hardly depends on class. I admit that 'route calculation' requires fast reading data from SD-card, but how 'writing to SD speed' can influence operation which not require writing?