Go download the actual Official Movies from Gamespot, then you'll see it in action. One thing I was impressed about was that the cannons had the ability to not only instantly eliminate infantry units, but also assert ragdoll physics onto them.
As for AOE3 being CPU intensive, that's arguable. This time the game isn't based on 2D sprites but a proper 3D engine for the vast majority to the game elements. You won't need one of the new generation video cards like the ATi Radeon X800 or the nVidia Geforce 6800 or the even newer (soon to be released in a few days) nVidia Geforce 7800 GTX but I'd certainly recommend atleast a Radeon 9600 or a Geforce FX 5500 (or better). Again, that's my personal estimations, we'll have to see when the min. system specs are released from Microsoft.