I think it's more efficient to have more memory clips for the same total amount of memory. I think they work faster this way. Makes sense, in a way. 2 GB of memory can either circulate through one port, or through two ports. Two ports, more room for data, or something. Also, I think they're supposed to work together.
Apparently, in pairs, they have more potential. So I think ideally, you should get 2 x 2 GB. Then you'd have four of the same. Or so I was told. I think. I'm not sure about those two first paragraphs.
Also, I'd like to add a comment about DDR2 memory. The demand for it is decreasing rapidly, which makes it expensive. Sometimes, illogically, you'll even see 2 GB of DDR2 more expensive than 2 GB of DDR3, for example.
Considering that more-than-decent motherboards with DDR3 capability and much more are not so expensive (my ASRock 970 Extreme3 cost me 150 $), if I were you, I would consider upgrading the motherboard and getting DDR3 memory. You'll pay a bit more, but I think it's definitely worth it.
Edit: Oh, riiiight, that's what Hennes said. Dual channel memory. Two of the same memory clips working better together. I knew I wasn't crazy.