I read a couple of benchmarks sites that I just punched up on Google and 5% seems to be accurate. Of course that's for gaming. DDR3 will be of more benefit in CPU and memory intensive activities. The mobo's chipset also factors in to all this.
It all depends on what you're doing with the system. I'd say if you're mainly gaming, stick with cheaper DDR2 for now. Of course it always helps to build your system to be futureproof.