what CPU and Mobo\Chipset do you have?
if your CPU and Chipset only support 1333mhz then the 1600mhz ram will just run at 1333mhz which is fine.
I would suggest buying the ram, since you only have 4GB now, 8GB will make a nice difference, also it will run dual channel (as long as you populate your ram sockets correct) so you should see a nice improvement.
Unless your CPU and mobo\chipset support 1600mhz, then you could replace your current 1333mhz ram with dual channel 1600mhz RAM, but the performance increase isn't huge, maybe 4-5%