You cannot say for certain one way or the other about the ram. Obviously if you add 400 ram to the 333 then the 400 will run only at 333. This is to match the stick that is already installed. The main issue is will the motherboard accept two different speeds of ram. Some do, some don't.
To be quite honest I would of thought that either you buy 333Mhz ram to match the first stick or sell the 333Mhz and buy two sticks or even better one larger stick of 400Mhz ram.