What do you plan on doing with this car. I have the same car and the two speed is probably the best upgrade. If you're planning on racing it I would say STOP upgrading it and get a better car. Not that the RS4 is a bad car. It's a pretty tough car, I like mine and I don't plan on getting rid of it. There are just much more race ready and tweakble cars out there for racing. Now, if you're just using it for bashing and racing around parking lots and such, I would reccomend a new pipe and the threaded shock kit by HPI, The pipe with give it a little more when you punch it and the threaded shocks will let you tune the suspension a little more plus they look sweet.
Please realize I'm not trying to talk down about the RS4 here. I was just trying to race mine at local tracks and now I realize that it's probably not the most cometative car to do that with. Hope this helps.