I have a 3 yr old store bought gateway pc that will not fit a dual slot video card and for the last 2 yrs I have have been using an evga 8800gt ssc video card. Is it worth upgrading to the evga 9800gt sc video card or better off just leaving it alone?
Not unless you are getting from someone used for $50.
It is a faster card than you have, but not a lot faster.
Add to the fact that your PC is 3 years old, and a prebuild, the 8800GT is probably a good match for what you have. But a 9800 would be a slight upgrade, it's just not gonna be a lot, and not worth spending a lot on.