Changing from a Mountain Bike Cassette to a Road Bike Cassette
 

Will I notice a significant difference in speed, etc? I have a Gary Fisher Big Sur 2004, I only use it for commuting on the roads. I changed the tires and the next is the cassette and chain, if it is worth it. Let me know!

not unless you pedal faster.

A road cassette usually has a tighter spread of gears, making it easier to maintain an optimum cadence. It might be a little easier to maintain high speeds with a new cassette, but it won't make too much difference. When it comes time to replace yours, there is no reason not to try, but don't expect miracles.

The new cassette will need the same number of cogs as the old cassette to be compatible with your shifters.

If you cant spin your feet any faster in top gear, you could get more speed by putting on a bigger chainring. I wouldnt know if that is possible with your crankset or you may have to change to a road crankset and have something like a 48T big ring.