First check your motherboard's specs and/or the User Manual/Guide to ensure that the desired RAM configuration ( 4 x 2GB) is indeed supported.
Overall, adding memory does improve system performance but if there is some other bottleneck then all will be for nought.
What games do you play? Go to the appropriate websites and check the games' respective requirements: hardware and software. Usually listed as minimum, recommended, and best. You do not want minimum.