Unfortunately, there is no standardization as to what the "x" rating means across brands of cards. Kingston states that "x" refers to write speed, where x= 150 KB/sec, so their 120x card has an 18 MB/sec. transfer speed. But other manufacturers don't necessarily use the same formula.
With SDHC cards, the class rating doesn't have a translation to that mystery x value in non-HC cards. But the higher the class, the faster the card-- and there are at least specs for classification of cards by class .
-------------------- Lisa Gade Editor in Chief, MobileTechReview