As Blam and Batman have already suggested you will not go faster.
The main reason is that your top speed is determined by the number of teeth on the smallest rear cog.
The smallest cog available on a normal cluster / cassette has 11 teeth.
If your smallest cog already has 11 teeth, then you will definitely not go faster with a new one. If it currently has 12 or more teeth then a new cluster can help you go faster.
But the main way of going faster is simply to pedal faster. With practice, you should be able to pedal at 90 to 100 rpm. I venture to guess that this is at lease half as fast again as your current pedaling speed.
The problem with pedaling slowly but getting the 11 tooth cog (if you don't have one already) is that you'll put a lot pressure on your knees and back, which often leads to pain and injury.
The advantage of switching to a higher number of gears is that the steps in between the gears is smaller. That can help you find just the right gear for the situation. But it doesn't change your top speed.
So my recommendation is to save you money for now while you practice pedaling faster. See also questions tagged cadence, especially What is the optimal cadence?.