You might get some lag if you are using wireless because you will have to first stream your data to your laptop and then from your laptop to the Apple TV.
I've got the same set up working without any issues with my old Apple TV but my iMac and Apple TV are both connected to the router by ethernet cable.
I have about 600 movies on a 640GB Western Digital My Passport Studio external drive and it works flawlessly thru the USB on my Airport Extreme hooked up to the ATV2 via ethernet.
Couldn't be happier......running at 720p the video quality is great.