Quiting applications frees up the RAM you need to play. To take this to extremes, you may even want to restart your computer before you start playing. This way, you can be sure that there is no memory fragmentation. In addition, other applications sometimes have extra processes that run in the background. You can't have that when you're playing games.
Games can never have enough memory.
The more you give 'em, the faster they run. You'll see a window similar to the one shown below. Simply boost the game's memory to whatever you want. There is no right or wrong amount, just remember that the more you give your game, the faster it will run. Just remember that this is not something for the faint of heart.