dual channel memory sticks in four slots

So I have 1 gig of corsair memory in two 512mb strips on my ASrock 939 mobo running on dual channel (DDR PC3200), whihc has 4 RAM slots in all.

I would like to upgrade and utilise the remaining two mobo slots for another 1GB, or possibly more. The mobo manaul insists that if I want to use all 4 slots I need 4 identical memory strips. Surely I can get away with another pair of strips without them being absolutely identical to the other pair? The phrase 'dual channel' implies this is about pairing up strips rather than a quadruple matched set ... ?

No problems at all.

I run 2x1GB g.skill and 2x512MB corsair perfectly fine in dual channel. You might have to relax speed/timings though depending on your processor - it should be fine at 2T cmd rate and DDR333 to be safe, then experiment if you want to go faster.