There is no point in changing shared. With 8gb ram, you probably have like 3gb shared on top of the dedicated amount which is 320-640mb depending on which version of the 8800gts you have. Most likely if you are having issues, the gpu being weak is the issue.
Dedicated is what's on the gpu, they are literally ram chips on the card and are typically faster then regular ram. Shared is the amount set by windows and is not configurable. It's typically around 1/3 of total ram so can change if you add ram. This is from system ram which will be slower so give less performance once you run out of dedicated. You can check how much is dedicated and shared from dxdiag or right click desktop>screen res>advanced settings.