Yes, i know its not going to be smooth, ive never said it would be smooth? Scroll up and look for where it says i mentioned it will be "smooth", because i never said it would.
It will be "playable". I have a helicopter pilot whom i have no problem with being my pilot despite him having a Socket 775 Dual Core CPU and a GTX 9800. Im not even kidding. He plays at low settings at 1080p, its not 60fps smooth; but he gets roughly 20-40 fps (depending on the map), if hes playing a "not as CPU-dependent" game -- so id say a population of a 15-25 person game. He flys perfectly fine; in fact my highest kill streak with him was 53 kills. We easily dodge through flying low into very small clearances between buildings; he honestly has the skill to be able to be playing at those low frame rates.
He has a 9800. That is WAY lower than a 550ti. Believe me. So its up to OP if he can play at such low frame rates/low settings considering he already games with his 550ti and even having a secondary monitor. Honestly its hard to tell right now. Basing results off Alpha gameplay is really not a good way to benchmark the potential of a game. The alpha is nowhere near ready to be even called a game; some textures and features of the game arent even in the game.... drivers havent even been released; like its really hard to judge the potential because sometimes a driver update could make the difference between "unplayable" and "playable" and possible "playable" to "smooth".
It clearly is just all speculation. For OP, Low... possibly Medium (considering that Alpha is not final release and driver support sometimes does bring really good results).
That is just my