I could be mistaken, but I think the Y510p uses Optimus, so when your 755M is not in use, the system is employing the Intel graphics. That should be where you're seeing some of the loss. Windows 8 uses up a bit of RAM as well.
To have a 64 bit version of Windows, you'll need to perform a clean installation. You cannot upgrade from 32 bit to 64 bit without a reinstall, unfortunately.