I don't do video editing, but I would expect that more RAM would perform better than faster RAM for that application.
As far as the games go, it kind of depends on the game. Most FPS games I think would perform better with 1GB at DDR400 than 1.5GB at DDR333.
You could try and see if you can force the memory to run at DDR400 in the bios with 3 dimms. May work though I doubt it.
Another possibility is trying to overclock the system. Since you are being forced to use a memory divider, the RAM won't hold you back. My 2.8Ghz P4 with the Intel 865PE chipset will run 3.5Ghz easily, but I have to use a memory divider. If I run with matched memory speed I can only get up to about 3.15GHz because my memory isn't stable beyond DDR450.