What to upgrade to achieve a higher top speed? I enjoy going fast and my current bicycle tops too early, what kind of upgrades would I need to make to achieve a taller final gear?
Bigger tires, bigger chainrings and a smaller cassette all lead to a higher gearing (the crank arm lengths among other things also come into this, but that encapsulates into bike fit). 

Bigger tires are subject to frame clearance and feeling squirmy possibly.

Bigger chain rings require front derailleur compatibility. 

Smaller cassette also requires appropriate derailleur capacities. 

Other ways of getting more speed include improving your aerodynamics (part of your bike fit - if you have drops for example, you may not be using them effectively) and increasing your cadence (most of the time when people ask this, their cadence is too low and they should be riding in a lower gear or pedaling faster). Losing weight (bike and yourself) does help as well, to some extent. If you're spinning out on say a steep downhill incline, that's normal (chances are you shouldn't be going faster than that anyway). 

You may find that the type of bike you're riding is not a good fit for your riding style, i.e. you're using a mountain bike for road riding. Switching the type of bike (which does include corresponding gear and position changes) can help as well.