Originally Posted by Little Darwin

You should be getting over 20 mph at 80 rpm pedal cadence (which is a completely reasonable cadence for most riders). If you are riding faster than that regularly, gearing might help. otherwise I would work on spinning instead.

If I am on a descent and hit my max (I am running a 44 tooth large ring, and an 11 tooth sprocket in the rear, which gives me a high gear that I rarely use) I enjoy the coasting... I actually spend 90% of my time on my 34 tooth middle ring, and only use the large if I have a good tail wind, or I am going down hill.

With a freewheel, you probably aren't going to get a smaller cog to get any higher there. And depending on the specific front derailleur, simply replacing the crank or chain ring could get you a higher gear. Going with a cassette would mean a rear wheel replacement, and searching for an evasive 7 speed cassette...

But, if you aren't regularly topping out at 20-25 mph, the best upgrade is your cadence.