Verify that they are both at DDR400, if so it should remain 1:1 (which is better as bagg said). If you run two sticks of RAM with different speeds or timings, they will automatically adjust to the lowest of the two.
Example, if the second stick is actually only DDR333 then the Kingston will be clocked lower to match. Same with the timings, if the new one is 2.5 and the Kingston is 3, the new one will run at 3 as well.
You may have to adjust it in your bios. Sometimes the rated speed or timings aren't automatically detected properly or are clocked lower by default for max stability and compatibility.