This class 6 micro SD (from Amazon) is selling for only USD18. I think other class 4 is probably selling for more than that. Your N900 will definitely benefit (cost advantage) with a faster write speed micro SD. Though I'm not sure about the N900 write speed (functionality benefit).

The Speed Classes define three minimum sustained write speeds in next generation SD Cards. Class 2 supports a minimum write speed of 2 megabytes (MB) per second*; Class 4 supports 4MB per second; and Class 6 supports 6MB per second. The SD Speed Class is shown as a "C" with a numeral inside, on the card and its packaging, above and to the right of the capacity rating.

Edit: The threads here have been referring to micro SD. Thanks Petur.