Forgive me if you already know this stuff, I'm going to proceed being extra explicit.

Next time you are out on your bike:
Get into the highest gear (that's the biggest ring up front and the smallest in the back).
When you get to your top speed, count the revolutions of the crank for 15 seconds. (For instance, the number to times your right knee is at the highest point)
Multiply that number by 4. That's your cadence.

One of the more common mistakes that new cyclists make is that they push too tall a gear at low cadence. Most people will find their most efficient cadence to be somewhere around 70-90 rpm.

As I mentioned before, at a cadence of around 80 in top gear you will be doing around 21 MPH on your bike.

If you are going slower than that, then your cadence is probably too slow and higher gearing isn't going to make you go any faster. You just need to ride more and get your muscles conditioned to bike riding.

Sorry to beat a dead horse with this stuff. I just think it would be a shame for you to spend on changing gearing and not go any faster in the end.

For more on cadence and gearing, take a look at Sheldon Brown's site: http://www.sheldonbrown.com/gears.html

Hope that helps...