Yup, if you run off one stick, you lose the DDR. The sticks have to be the same size, and even model. Two GB sticks works.
You don't 'lose the DDR'. You lose the ability to utilise the dual channel architecture (i.e. the use of two 64-bit data channels).
A single DDR-SDRAM module in a non dual channel configuration will still operate at it's rated DDR clock speed, but will just use one 64-bit data channel instead of two. In other words, the clock speed doesn't change, the bandwidth / throughput does.
In context, a single DDR2-400 RAM module (also known as PC2-3200), on it's own in a non dual channel configuration, will operate at 400 MHz giving a peak transfer rate of 3,200 MB/s.
Two DDR2-400 RAM modules in a dual channel configuration will still operate at 400 MHz, but instead will provide a peak transfer rate of 6,400 MB/s, due to the utilisation of two 64-bit data channels (a total bandwidth of 128-bits).
http://www.hardwaresecrets.com/article/133/5
Back to the original query, if your motherboard supports dual channel configuration, then I suggest taking advantage of it by purchasing and using two identical RAM modules.