you can't change the speed of a microSD card... the speed is affected solely by the quality of the NAND chips in the card and the overall build quality.
Watch that spam DQ! and yes when you are looking for mSD cards it will tell you what the speed is, generally kingston and sandisk are the best, but most branded mSD will run most games, its usually those cheap ones you gotta worry about.
That goes for cheap branded ones too, especially coming from places like ebay, because there are some convincing fakes around.
At the Netherlands the Sandisk and Kinston are so low priced as 14€ for a two gig. card, not worth it to buy a cheap one. Don't know how much they cost at other places, though.
Sorry about that Almo, but mostly everyone here has a Kingston Micro SD Card right? Because I dont know if mine is screwing up the R4
I have a Sandisk too. Some people say that Kingston and Sandisk are the same while others say that Kingston is better (meaning Japanese made ones).