Some boards which offer dual channel RAM access are rather touchy with the brand and speed of memory sticks installed, but matched pairs are not absolutely necessary. However, a matched pair has been tested in tandem prior to packaging, so one has a bit of a leg-up there!
Remember that the sticks need to be in the designated slots on the board, as described (hopefully) in the manufacturers manual, to run in dual mode.
I already have one stick of 512 Pc3200 Corsair memory on my comp. I wanted to buy one more so i could have dual channel, but the sales guy said i would have to buy them in pairs that say they are dual channel memory. Is he right or just trying to make a sale?