I believe and i'm not positive but if the options are only to 128, that is it, but the overall performance should be really good.
You can try, but if you are doing a fresh install why not configure it for the new chip
And for question #3, booting shouldn't have anything to do with the graphics card, but a while back i was trying to install a distro and the graphics card in this old machine did give me probs and not let me install, but with a new card it shouldn't be the card keeping u from booting. I'm sure there will be some more responses to this post.
Sorry i couldn't help more