I'm possitive that the 333 and 400 are Mhz clockspeeds for RAM, although I could be wrong but I don't think I am, anyways if you think about it the DDR333 is slower then the DDR400 as the DDR400 is better so yes it will slow you down but's its more to the ponit will you actually notice the performance between the two? I'm sure you wont - I think if you are going to notice the proformance is the timings on the RAM. You should also consider buying the same amount of RAM, from the same manufacter and exact RAM sticks as this allows dual channel and also some motehrboards have problems dealing with two different types of RAM.