Nope, you wont. If you keep everything stock, then when you buy faster RAM, the timings are also higher, which pretty much get rid of anything you would have gained.
RAM speeds arent the bottleneck anymore, but if you were going from say DDR266 to DDR400, then I would say yes.
And as cromewell said, the only difference you might notice are in benchmarks such as PCMark or SiSoft.