Why are you removing GregTech? This started out as help for textures in a mod to help removing and adding mods.
I'm removing gregtech because of its craftings.
Some of the files minecraft needs apparently have not been downloaded. Remember to backup first. If you have another modpack you should be able to copy over your resources folder to this modpack. Or try force updating the modpack.
Not really sure, but you may have to click OK or Yes when the world loads. I have not removed or added mods after I started a world. Remember to backup when ever you do something like this.
It is often suggested that FTB runs best from the desktop in its own folder. Installing from there seems to solve a lot of problems for some users. If you are up for reinstalling the recommended way please follow this
guide exactly.
Edit:It did not seem to keep part of my post.
You have a 64bit OS you should be using 64bit Java. This will also allow you to allocate more than 1GB to the launcher and may improve performance.
I can't because my ram isn't enough, with 1 gb sometimes doesn't start S: (I've 4 GB but with lots of processes and the ram is only 665 MHz frequency)
Edit2:
This is the latest version of Light, but you might want to try Standard or Ultra. Ultra will allow you to use more than one core of your CPU for minecraft if the option is selected in the menus.
Standard\Ultra version of optifine?