Yes, if you want a noticable change go 1 tooth smaller on the front, this will lower the gearing and alow you to come outa corners better in 3rd gear instead of 2nd making for gaining speed quicker. Going bigger on the back will also lower it but the back sprockets are higher if you go with a Renthal.
I'm with Rich, but also make sure it's healthy to start with! A good topend, reeds, a properly packed silencer, and a fresh clean air filter for starts. No use in fiddling with anything if the rings are shot and the reeds won't close.
What kind of riding do you do? I would suggest a properly built suspension long before power mods. A 250 will pretty much make more than enough umph for anything short of dune blasting.