Depends on the project. You probably want to match it to the project's resolution, unless you intend to zoom in or pan across or do anything else which requires moving into the picture.
I do a lot of work in Vegaswith still images. The rule I've found is to try to stay at greater resolution on the source than the cropped area you are attempting to show. In other words, try to stay greater than 1:1 on the resolution.
bsmall is right, DPI has no meaning in this context. It's all about the number of pixels in screen space.