AA is a graphical setting that in simple terms smooths the edges of objects. This is entirely personal preference and depends on the hardware you are running. Some people prefer to turn up the graphical detail and leave AA off or at a lower setting. Other prefer to turn down the graphical detail and turn up AA to smooth the edges. If your hardware is capable of running high detail and also a high AA setting, that is usually optimal. But in no way is it necessary. Since you have a GTX480 I would go for the gold and CRANK everything up and go from there. You may find some very demanding games that you need to turn down the AA settings or detail. In the detail settings there is sometimes a high, very high, and ultra high. Some games incorporate the ultra high for people running insane systems with more than one GPU that can handle this type of detail. However, in most cases your card will max most games.
Personally I usually enjoy a higher detail before I enable AA settings. See this link for some more info if you like.http://en.wikipedia.org/wiki/Anti-aliasing