Dual channel: 2 different sets of matched ram OK?

My mobo (abit IC7) has 4 slots. Currently running 2 identical 512mb sticks in 1 and 3. Dual channel enabled, works fine.

Would dual channel still work if I bought another matched set to run in slots 2 and 4?

So I would have a matched set in 1/3, and a different set, yet matched to eachother, in 2/4.

Does the second matched set have to be identical to the first (timings, module size), or is the only requirement that each set must be identical to eachother?

Ideally I would have 2 512mb in 1 and 3, and 2 1gb in 2 and 4.

thanks.

Yes, you would still have dual channel runing 4 sticks. Some mobo's will only allow certain speeds with 4 sticks in dual channel. You probably won't see any kind of performance gain in overclocking doing it that way. You would be better off getting 2x1gb unless you want to run nearly stock.