If you use your BIOS's Default or Automatic RAM settings it will run at the slower speed, but you may be able to make the 333 run at 400MHz by changing the setting to User Defined and setting the speed manually.
I did it to make my old mismatched ram run at the same speeds. I had 1 stick of PC2100(266MHz) and 2 of PC2700(333) that all ran at PC3200(400) speed. Of course this will all be dependant on the options your motherboard has. If it's a Dell, HP or other name brand, prebuilt unit your options may well be limited. If it's a "custom" build then you should be good to go.