O.k so you have onboard graphics and you added in an 8400GS?
Anyway it has nothing to do with your GPU. Your CPU is slow. that's all there is to it. Flash isn't too efficient with CPU cycles (though it's better than it has been), especially if the programmer is just trying to get it to work. After all, a low end target machine now would be an original Core2Duo or an Athlon X2. Developers aren't testing their flash apps to make sure that people with old Celeron 3.2Ghz machines are having a pleasant experience. You add the fact that many sites, like this one, often have multiple flash adds and you get a crummy web experience. So, don't open as many browser windows/tabs, or upgrade your CPU, like now.