Purchasing anything new at the moment will not solve any of the current problems. In fact, there is a "newer" card that is causing headaches. New is not neccessarily better for this game. The key for avoiding lag is not to have over more cities, than your computer can handle. My computer is below the minimum in every aspect and the game plays just fine, as long as I do not go over the threshold. If your computer starts to lag, does not matter which turn, you know you have hit that point.
The demo was not the vanilla version. The demo was actually a version, before they chopped everything out and forced an "early" "steam" enabled version that was not ready when the original release date was "kept". Instead of getting all the steam related bugs out, the game was released "on time", and "we" did the testing ourselves.
Yes it is true that the demo will not (unless you are really good) get to the point where 20+ cities starts to bog your game down. Most people who like quick small games do not have trouble at all. If the demo loads and plays for you fine, then the game itself will play for you just fine also.
Graphical settings surrounding Direct X11 has issues with the newer generation of GPU's. If the demo works fine on your current equipment then you should enjoy the game just fine. If you need a newer computer that will help out in other areas of your life, then get a newer computer. Spending top dollar for this game is not advised, until all of the bugs are fixed and cutting edge graphics become more mainstream with all of the bugs related to it "fixed".