I have a D60, which has about the same write speed as your D3000. I also have a Transcend card (old Class 6 model, but it benches out to 10 MB/sec on Robgalbraith.com). I also have a Sandisk Extreme III (30 MB/sec) model. The Sandisk is a faster card and the buffer clears much faster, allowing me to take almost double the number of shots in a minute of time - shooting RAW.
However, there is a limit to how fast the buffer will clear. Nikon did not put a very fast chip in these entry level cameras. For instance, I find that my D7000 would write much faster to the same cards than the D60, because it had a faster processing pipeline.
robgalbraith.com has some info on SDHC card write speeds with different models of cards. The info is about D7000 speeds (he also has a table for the D90), but it should be useful as you gauge relative speed. http://www.robgalbraith.com/bins/camera_wb_multi_page.asp?cid=6007-12454
Similarly the SportsPhotoGuy.com website has the info on quite a few cards in the D7000, so you can gauge the relative speed of the cards. http://sportsphotoguy.com/nikon-d7000-raw-burst-test/
Hi, agree with the comments here, I noticed a significant improvement when I went to class 10 Sandisk Extreme with my D90, see if you can try one in a shop somewhere before you buy, cost a bit more than class 6 or below.