My opinion is that the class 2 SD cards are already faster than the N8x0 memory card interface can use. There is/was a 48MHz (?) kernel patch that theoretically doubles the N8x0 read/write speed, but it is not supported by Nokia. Only if you are using this patch then Class 2 cards may become bottleneck.

You should also remember that the classification only refers to the guaranteed minimum sustained write speed. Read speed has nothing to do with this, and is typically much faster than write speed.

Also the Class ratings can be conservative: I have couple class 2 Sandisk 8GB microSD cards, and both can write over 6MB/s when using external card reader. When using with the N810 the write speed was significantly lower... between 1-2MB/s if I remember right.