You are correct about how to create graphics in Photoshop CS2. My suggestion would be to use the Photoshop preset for new documents called...
HDTV, 1920x1080 (with guides)
Tangent begins here...
dpi is not relevant to video.
Short story...
It is because people could watch your video or film production on a five inch screen in the back of a mini-van or a forty six inch plasma in the basement.
Long story..
You choose the number of pixels your viewer sees once – regardless of the size of the screen they watch it on.
That choice is the delivery format. The choice of delivery format is related to the choice of acquision and editing formats. Each should inform the other.
The other consideration besides the number of pixels is compression – but this is not directly relevant to why dpi is not a real number for video production.
For instance, you might shoot HDV, edit in DV, and output to DVD.
This is two different pixel resolutions and three different compressions. However if the work is thoughtfully done, these images can work well regardless of whether it is presented on a 5" screen or a 42" screen.
dpi is a print term because In print you generally know how large the output surface will be. Because the image does not move in a print image (as opposed to a video image) their is also less to distract you from low resolution.
dpi combined with the size of your output determines the number of pixels in a print document.
The .tif file you send off to the printer go get business cards made would not blow up well to be printed on a billboard. That would require more pixels given the same dpi.
Does this make sense?
Tom