You could also try installing the GWX mod and running the "Black Sea Missions" (or Indian Ocean) only. This will allow you to only operate within a smaller area but the upside is that the game will not be running all the rest of the combat areas. Although I haven't tried this myself it is described in the instruction manual that you get with the GWX mod' and should speed up the load up time too.
I am currently running SHIII with GWX on a machine that is about 9 years old. It can be a bit framey at start up but I can just about get away with it.
Good luck.
