You have know what DC does. Once you do, the answer is pretty obvious.
Do a little research. It'll be worth your time.
Essentially, the latency across two large sticks like that (1gig) is going to more or less negate any advantages of DC. That said, it will still be worth your money to buy the second 1gig stick, because 2gigs of RAM would be good. As long as the RAM is fast and low-latency on its own, you have little to be concerned about. DC, if you use it, will just be an extra bell and whistle.