Yeah man. When you get dual channel, you'll have maybe four slots. Two slots go to each channel. So yes, you could have 2 sticks of 512 mb in one channel, and another 2 sticks in the other channel.
Sure... I've heard anyway, but 4 sticks of 512 vs. 4 sticks of 1 gig... I think the 4 sticks of 1 gig would win, but realisticly, nobody needs 4 GB of RAM right now.