Most all the newer furnaces there will be a 'Y' terminal at the furnace. However, this 'Y' terminal does nothing at the furnace location. On the board, it is a connection point only. Essentially, the Y signal originates at the thermostat, travels on the yellow wire, passes straight through the furnace out to the starting contactor at the A/C condenser. That contactor at the condenser also needs the B/C from the furnace. These High tech thermostats will also need a common, B/C wire as well as the normal control wires.
Better insulation is always a good choice, though not always the most cost effective.
The cheapest solution, is to adjust the registers. Use the registers to reduce the flow on the first floor, and increase it on the second. This may require closing some of the first floor registers completely. This solution will likely not provide the best results, but may make enough difference in your situation. And besides, it's free.
If you have a little money, and the tools and knowledge to work with ducts. You could install dampers in the ducting. If all the ducting is exposed in the basement, this can be an easy job. Once installed, you can use the dampers to direct the conditioned air where you need it.
If you have a bit more money, you could convert to a dual zoned system. This will require installing dampers, a zone controller, and additional thermostats. This will allow you to control the temperature on each floor independently (sort of).
If you have lots of money, you could install a separate system for each floor. This may not be practical in a smaller home, but is quite popular in larger homes. You'll have a separate furnace and A/C unit for each floor, which are controlled independently of each other by their own thermostats.
Best Answer
Zone system
A true zone system allows different areas of the house to be isolated, by closing off certain areas using dampers, or just having completely isolated air handlers.
This allows fairly accurate control of the temperature each zone, allowing a specific (and if desired, different) temperature to be maintained in each.
Multiple sensors
The Ecobee is more of a "multiple thermostat" or "zoned sensors" system (not sure if there's a more proper name). The limitation is there is no isolation in air handling, so multiple zones cannot be controlled independently.
However, there are two distinct advantages over the typical "single thermostat" setup:
1) The house can be maintained at a temperature based on averages of all sensors
As an example, let's say right now the upstairs is 26 while the main floor is 21, and your target is 21.
By averaging everything, it'll keep the house slightly cooler and set the upstairs to 23 (a bit above target) while the downstairs is 19 (at bit below target).
2) Which sensors you're using can be scheduled
This means during the day, you can give priority to the main floor living spaces, while at night you give priority to the upstairs.
The downside is during the day the upstairs floor is uncomfortably hot, and during the night the main floor is colder than desired, but the areas where people mainly are stays at a nice temperature.
Aside, I have an Ecobee3 and this works extremely well for me. I also use the "ensure fan runs for at least x min/hour" feature which recirculates air and (I think) keeps everything more even.