Ok, if you put two different memory sticks (in speed) then the will run at the speed of the slowest ram. So typically it will run slower. (depending on the speed) It might not be a dramatic drop.
For example: Say you have a
1GB DDR2 @ 667Mhz
1GB DDR2 @ 800Mhz
If you use them together they will BOTH run at 667Mhz. So it will run slower, but it would still run fine.
From this point you can overclock the ram speed back to 800Mhz so it all runs at that speed. But this is only if you OC, otherwise it stays at 667Mhz.
does that help?