So, will the Nokia N9 be able to run ICS smoothly? Or will it lag like the N900 running ICS? With the specifications of the N9, it should be fine though (if everything works).

Developers have managed to get ICS on Samsung Galaxy S, which has approximately the same specifications as the N9 (Galaxy S, not SII).

With Android on this device, it basically should be Nokia's best phone. To be honest, it probably already is. Nokia rarely release any phones with high-range specifications, other than the Nokia Lumia (which is still not high-end). So N9 is basically the phone to get if you want Nokia!