As a recommendation, I recommend going with a 2x 1gb kit. What is your budget??
DDR2 prices have plumetted over the last year. There are great deal to be had.
You want 2 of the same ram modules to take advantage of dual channel.
As the above poster mentioned; you put the in the same color slot. Ie, both in blue, or both in black, or both in yellow.
Depending on the motherboard manufacturer, they may be touch slots, or the colors may alternate. I don't know why they don't all do it the same; but big companies don't always keep things simple or make sense sometimes.
For example. Here is a nice budget set. Extreme budget. I don't know what you're working with. but $24.99 for 2gb dual channel is not bad Newegg LINK