When looking at a HDV clip in vegas and if the motion is a bit stuttery or choppy, is that the video card or the cpu? What is the culprit

I've got a 3.0 ghz p4 running vegas 6.

I would say it's Vegas 6. Upgrade to Vegas 7 and you'll see a very marked increase in the playback framerate. My laptop went from around 4-6 fps in Vegas 6 to 25-30 fps in Vegas 7.