Try using the Lite Harbour Traffic Mod that is included with GWX.
GWX has mad nay improvements to harbour life, including better harbour textures & more shipping.
I could run GW with a ATI 9600 128mb card (no bells version) and maintain 15-25 fps while in port. This was with all AA & AF options off. There should be no reason for you to strugle with GWX as GW was more of a strain to your system.
If you think memory might be your problem then increase your pagefile size (aka virtual RAM) it might help.
FYI - Kiel is the busiest port in GWX, get it sorted here and you'll be alright for the others.
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