How do I run PC based games faster on my mac through Parallels?

So i have this game called Rift that im playing through my Parallels program with Windows 7. i was talking to my older brother about it and he said it had to do something with re-arranging the Memory used. is this true? (if so where do i change the memory used and to how low or high/ On my Mac or my Windows?) Also is there any other way to make the game run faster other then setting all the graphics low because i have done that.