Your computer isn't good enough for the new coming Settler VI, which is a pity as it will be a great came (I've seen some trailers, and I am really going to buy that game).
Unluckely Settler V is nothing other than an ordinary RTS.


Anno 1503 on the other hand might be of your liking, not the newest game but it certainly has a good city builder aspect. Little diplomacy but also some military.

Or you could try Patrician III as Froggy said, or Port Royale II. Haven't played those myself, but heard that they are good


Linky to Anno 1503:
http://www.anno1503.com/english/index.php4?language=2