You can't desable overworld. it is part of vanilla game. If you don't want to travel in space, you can disable "spawning" rocket in other dimension. I am on my netbook now so I can't tell you where is it till Monday. Simply take a look in your main config of gc.
You have nvidia graphics that don't "like" java. Try to use bit-nubers (2-4-8-16-32-64...) for the ram. If this won't help, re-install java and graphics drivers. Btw. I have 2gb of ram. I'm using 1gb for galacticraft and works great.
Sometimes config-mod-update causes crashes. Try to delete your config folder. If that won't help update your forge and delete and download new .minecraft.
Ok, found something... if is meteor spawned, makes blast attack <30. Tin Dec. Blocks, Sone, Cobblestone, Mars blocks ect. have blast resist 30 so meteors can't breake it. So meteors can breake block with <30 blast resist. (dirt, moon turf...)