Any increase after 3.8ghz~ you really won't see much of an improvement in gaming and probably minimal change in performance from bumping to 4.00ghz. It's not really for "extra performance" as much as it would be testing your limits.
You raised the multiplier to overclock so your memory remained intact. You can overclock your ram too from increasing the CPU bus speed if you wanted to. But the performance gain in my opinion is not worth it.
Your ram just like your chip has it's limits. Often times you have to downclock you ram in order to continue raising the CPU bus speed. So you'd go to your bios and set the ram to like 666 MHz and due to the overclock from the bus speed the ram is still operating at like 800mhz or something.
Extra ram/unused ram is really just a waste of money. Unless your creating 3d models or something memory intensive, then you don't require much more then 4Gb.