What I want to do is upgrade my ram by replacing 2 of my 1 gig sticks with 2 4gig sticks. But I'm not sure if I can mix and match ram modules like that. I know they need to be the same mhz so I need DDR2 800s, but do they need to have the same timings and voltages?
Isn't necesary that the RAM been the same MHz, if you install 1066 and 800, all RAM run at 800. Is better a low voltage ram, maxi 2.0V.
For your answer, yes, you can mix the old RAM and the new one. BUT maybe you don't see any diference in performace, you old RAM has CL 4, the new have CL6. If you buy the Kingston, set the voltage of your old RAM to 1.8V.
There could be a problem since you specifically say you want to overclock. I'll guess you mean the ram as well as the cpu. Certainly the 1066 will have plenty of headroom, it might be interesting to see what latencies the 1066 chips have programed in them by the SPD settings.
I would thing you'd want to lock in the settings to match the weakest parameters of the mixed pairs in bios to get your most stable overclock.
I could be wrong but my opinion is tied to your statement about overclocking.