RAM will run at the slowest speeds and lowest bandwidth available...meaning when you compare the chips. You're best bet, however is to match your sticks up, to avoid any potential compatibility issues.
The other thing to remember is, don't get too hung up on rated speeds and timings. Far too many people worry about them and pay extra for specific settings, without ever really seeing any kind of specific gains. We used to call them Benchmark Junkies...because that's all they really used their computers for.
I'm not sure about your motherboard, but you typically want them to be in matching pairs. I would consider getting a chip identical to your current 2 GB chip, and running them together to make 4 GB. You already have x64 Windows 7 installed, so you shouldn't have any issues seeing and using the memory.