No, you shouldn't mix and match ram like that since they will cause your system to become unstable. The best course of action is to populate all your slots with the maximum capacity per slot and if possible to have rams with spec's a notch lower than what the board's maximum is e.g: if DDR2 1066MHz is the maximum you should go for DDR2 800MHz sticks. Mind you, it's highly recommended that you update your BIOS prior to upgrading your rams(if the board supports it). Speaking of support, you should go through your motherboard's manual to take note of the maximum ram supported by your platform.
Might I ask what your full system's specs are inclusive of your OS?