If your memory supports dual-channel memory, then YES two sticks are faster than one. Dual channel memory allows the CPU to access the memory on two different channels, enhancing the bandwidth, giving you more speed. You can even run 2 dual channel sticks, which means 4 sticks total, but they all need to be identical, and the new Phenoms' quad-cores can theoretically access each stick individually. Remember, dual channel memory means that both sticks (or all four) have to be identical. This is why they're sold in matched pairs.
If you're motherboard doesn't support dual channel or the new triple channel memory architecture, then it doesn't matter what you do, 1 stick, 3, or 4, you won't see a difference.
EDIT: Cromewell beat me to it