DDR3 1600 should give you more headroom for overclocking, depending on how you overclock (either OCing the core clock or multiplier). But most benchmarks don't notice any difference in performance between DDR3 1600 or DDR3 1333.
If you are going to try and overclock down the road it might even be worth your while to find out what kind of IC's the 1600 or 1333 kits use. for instance some 1600 kits are really just 1333 kits that are a slightly better bin, and then the memory company just factory OC's them. and visa versa, sometimes you can get the same (but lower binned) IC's in a 1600 kit that the mem company also OC's to make it their 2000 kit.
keep in mind this may not make all that much difference if you are only going to tinker around with OCing.
My recommendation would be a 4gb 1600 kit like this: http://www.newegg.com/Product/Product.aspx?Item=N82E168...
because you can overvolt it to 1.95v (which should give you a little room to tinker)without voiding OCZ's warranty.