I don't know why all you are saying the gf4200 will beat a 9550, I don't think it would. A gf4600 will brobably beat it, but not a 4200. Anyway, a 9550 is basically an underclocked 9600pro, so if it's a 128-bit card and you OC it to 9600 speeds, it will likely stomp the 4200 in all games. As for dx9, I don't have the card so I can't say for sure, but it would likely be able to run dx9-enabled games, as long as you don't use AA and keep the settings at medium levels.
However, if you already have a gf4200, I'd save up more money for a 6600gt, or even a 9800pro, you can get those for like $130 nowadays. Both will give you a big improvement over the 4200.