First off. Welcome to Subsim!!!
Second, graphics cards may not be the issue. Can you give your CPU specs, OS, RAM, and current graphics card??
For graphics cards I run on a NVidia GTX 660M and get 20-30 fps in Port with mods and stuff and can get 40+ out at sea.
Buuut that is running a quad core i7 at 2.4 Ghz with 16GB RAM (Unessescary really)