There are two answers:
1) (snarky) If you don't know if you need 8GB of RAM, then you don't need it.
2) Use the Activity Monitor.app to monitor your...um...activity. You can see how much ram you are using under different use conditions. If you are on a 4GB machine a regularly use most of the RAM then 8GB will likely help. If you rarely exceed 2 to ~3GB then the jump to 8GB is not likely to be noticeable.