Delta is right, it doesn't affect the performance of your laptop at all.
However, there is another option where you wouldn't need to buy another monitor. I use a Kensington docking station and I love it. Makes packing up and leaving simple and yet gives me the desktop feel when I'm at home. K33239 is the model number.
i saw one like this at circuit city and the guy told me that using it would definately affect the performance of my laptop.
when you use yours do you have a monitor/printer/speakers/etc. plugged in through your docking station? im not like a big gamer or anything so pretty much my usage of the computer will be Word, Internet surfing and research. i just dont want to spend the money and have it be so slow that the picture on the monitor moves in like slow motion but i would prefer to use my larger monitor and full size mouse and keyboard while im at my place.