I just upgraded my Aluminum Macbook 2ghz to 8GB of RAM and while it seems faster its not as much as I was hoping for there are some quirky bugs in Mountain Lion and it forces me to have to dual boot Snow Leopard as well. Video performance in Mountain Lion is abysmal skips a lot and seems to have trouble reading off the drive, whereas Snow Leopard plays HD videos flawlessly in quicktime and I can even connect my TV and browse the web while watching a movie no flaws, but in ML all stutter.
Maybe I need to upgrade to a newer solid state or hybrid drive, but it works fine in Snow Leopard.
Mainly happens with large movies that are 6GB+ mp4, mkv, ect. And VLC is completely useless any kind of movie whether snow leopard, mountain lion, HD or not, it stutters, so I dont use it. Perian still works well enough.