I would strongly suggest an AMD R9 270/R7 370 (almost exactly the same card) whichever is cheapest at the time, or an Nvidia equivalent GTX 660/950, again, whichever is cheapest.
The AMD cards are cheaper, and the main reason I would suggest them is because of this and the fact that as a fellow a Anno player I can tell you that 2205 is VERY texture heavy, and the AMD cards tend to be better for this:
e.g. R9 270 Texture rate = 72 GTEXEL/s
vs GTX 950 Texture rate = 49.2 GTexel/s.
AMD cards also tend to have more Shading units, Texture mapping units and Render output processors etc.
(this changes as you get into the top tier cards though)
However, I don't know how big your power supply is and it should be noted that the older cards require more power:
R9 270 = 150 watts
GTX 660 = 140 watts
The newer cards:
R7 370 = 110 watts
GTX 950 = 90 watts
At 720p Anno 2205 will look gorgeous on any of these cards, bear in mind though that Anno pushes ALL graphics cards REALLY hard, and that lag you are talking about is caused by the immense texture fill.
A few other things: you experience greater "lag" as your metropolis grows, so bear this in mind late game.
Related to this, like all modern 4X/City building games, they prefer muti-core/-threaded CPU's, and this is definitely true of Anno 2205 (although only physical cores seem to make a difference, multiple theards make a negligible difference).
Seriously going from 2 cores/2 threads to 4 cores/4 threads increases framerates by about 60-70% which makes it one of the most a CPU dependent games I have ever played.
If you do plan on getting one of the above GPU's, chances are you will only notice a minor bump in performance due to your CPU being a major bottleneck in this specific instance.. Sorry man.
Lastly, if you decide to get upgrade your GPU anyway and maybe upgrade your CPU/MOBO later on down the line, and you want a best-bang-for-buck GPU then there is only one that I can recommend at the moment: the GTX 960, it's only a bit more than the above GPU's, gigabyte are doing a small form factor one that is an INCREDIBLY good price atm and it only runs at 120 watts. It really does give the best performance for the money.
Hope this helped, Enjoy Anno 2205 it's... Different (I try not to think of it as Anno when I play it, more like... Simcity with more resource management + Arctic and moon add on packs lol, it actually really does increase my enjoyment to think of it that way... Coz whatever it is, it ain't Anno.)