Matching DDR2 memory

I am a little confused about DDR2 PC26400 memory. I was hoping to hear some opinions on "matching" memory modules. I have 4 slots on my mainboard. I am currently only using 2 of them with OCZ pc6400 2gb (2x 1gb). I want to buy 2 more sticks , but am confused about what to buy. Should I get 2 more 1gb sticks or 2-512mb sticks.Would I be better off with four sticks that are the same??? Or 2- 1gb and 2- 512mb??? I will be buying the same brand and style.

I would try to get the same sticks... in the same size (2Gb total memory is good enough for most of us).

However with 4 sticks in your system you may find it more difficult to lower the times and OC the RAM.

1T timings are possible with only two sticks, but as I understand it AMD systems force 2T when using 4 sticks. The performance difference is slight though.

Thanks for the input