Those system specs are nearly identical to my old computer. My old vid card was an MSI GeForce FX5200 128MB.
I can tell you this much from direct experience...
That vid card of yours is the primary cause. First, the particle rendering bug in SH3 gives older nVidia cards a fit. You can start by turning the Particle Effects slider in SH3's Video Settings menu to 60 or less. Then make sure you have AA set no higher than 4x and AF to 6x in the nVidia Control Panel. 2x AA and 4x AF will speed things up a bit more.
The other inescapable fact is GWX 2.0/2.1 is a bit more than the FX 5200 can handle. I recommend you use the Lite Harbor Traffic mod for sure. Rendering all those ships will kill your FPS. Also, you'll have to live without the Water Streams and Exhaust Smoke mods. The same goes for the Lifeboats and Rowers. The less eye-candy that has to be rendered, the smoother your framerates.
As for those crashes, it sounds like a mod did corrupt something. That error message is the big clue. The
Tactical Green Mod alters the menus in game so there's your culprit. It seems some button or other doodad in one of the u-boat's command menus is missing and the mod may have improperly overwritten the feature referenced. Try disabling the mod and see what happens.
The Tactical Green mod may be conflicting with another mod that alters the menus, like OLC GUI or the GWX Integrated Orders mods. Check the mods' readmes and ask the mod creators if you have any issues.
Always make sure you never apply mods unless you're docked in port between missions. Unpleasant things happen when you alter files that are already in use during a patrol. The only exception to this rule is simple graphics mods that consist of just '.tga' files, like sub-emblems and skins.
Bear in mind that it can take up to 10 minutes or more for SH3+GWX to load on your system. So be patient and go grab a soda or make some coffee while you wait.
One last thing...NEVER EVER ALT+TAB out of SH3, unless you do need to look at an error dialog from a crash. SH3 doesn't like not having 100% control of your system and switching out to another task causes some unexpected weirdness during normal gameplay.
I hope all this rambling helps you.
