The class is less important than getting the right brand; certain manufacturers try to just meet the requirements while others far exceed them but only class their products to mid range prices.
For example, I have a verbatim branded class 6 and a kingston branded class 4, both 4G in size - the class 4 is around 20% faster on read/write benchmarking (same read, cards were both blank and freshly formatted for the test), as well the latency of the verbatim was significantly slower (on DS cards this seems to be the important part.) Off the top of my head, the read speed verb:14.2M/s, king:18.4M/s and latency verb:1.2ms, king:0.9ms - I didn't pay much attention to the write speeds at the time but I do know I can put large files onto the kingston much quicker than the verb. (btw: I got them locally on sale, the verb was $20, the kingson $17.)
Heck, I have even seen fake memory that exceeds the product specs of the original they are copying - in everything but longevity (they fail long before the warranty would expire on the official product.)
Maedhros: write speed doesn't affect the read speed, but it does effect the cards overall performance and thus it's rating (as mentioned above, one of their main target customers are those with digital cameras that can do things like high speed photos or video.)
