Idealy the should be matched, but we don't live in an ideal world, I am running 8gb 2 sets of 2 different 2x2gb, sure they may well both be geil ram, but they are different speeds (the slower set runs fine at the speed of the others with a little voltage help
), and am pretty sure they are different ic's
My brothers pc has some kingston value ram running in dc mode with some expensive crucial.
It isn't a nessesity to have them matched, but it can help with troubleshooting incase one of the sticks isn't fully compatible for some reason.
Edit: Oh and if you get 2x1gb, most motherboards will allow you to populate channel 1 with them and put the 2gb in channel 2 and it should work as dc as previously my brother was running 1x1gb and 2x512mb just fine.