Yes. It's absolutely fine to leave all Graphics Processor stuff turned off.
I think it should be on "all images". I don't think it's a good idea to have a difference between preview and output, if you know what I mean.
How many hard drives do you have on your computer? I have left my cache at 1GB, but made a cache folder on D drive instead, so it doesn't interfere with the programs running on C drive. I don't know how technically correct this is, but it seems to work ok for me.