Is the class rating of my MicroSD card relevant? Does Class 4 perform differently from Class 6, and so on, when it comes to Micro SD(HC) memory cards on smartphones? Samsung Galaxy S to be specific.
The general consensus seems to be that a Class 4 is just fine, and you definitely would not need anything faster than a Class 6.  Unless you're seeing a lot of lag during IO intensive operations, though, I wouldn't be worried.  The best way to test might be to take a few minutes of 720p video.  If it records and plays back fine &mdash; and it should &mdash; you should be good for anything. Since L3 video at 1080p is just over 5.5 MB/s, a class 6 card is sufficient.  The Galaxy S II only takes 1080p video at up to 3 MB/s, and I don't know of any phone at the moment that takes higher-quality videos.

Note that the Galaxy S uses the internal SD storage for most things (possibly including video; not sure if you can choose the save location).  So the external SD card may in fact be irrelevant most of the time.  I've had my Galaxy S nearly a year and used the external SD card approximately twice.

If you're copying large files onto the SD card, it will obviously take less time if the class rating of the card is higher.  This might happen on file transfers from the computer, or 4G downloads with low network usage, but not during general use.