While I agree "it depends on the project", I am specifically interested to know what NON-FULL-SCREEN display sizes people use (or plan to) for video embedded in a web page.
If you take a look at pages on "TED", you'll see the layout has been designed for both "classic" (4:3) and newer (16:9), "HD" video:
Older Video (4:3 aspect) Aug '06
Newer Video: (16:9 aspect) Aug '07
While 320x240 is "the standard" -- there is no actual standards body that declared it to be... 320x240 evolved from the fact that most users, due to bandwidth and graphics display capability can not view "full screen" video, (the machine won't keep up with 24+ frames per second full screen refreshes without stuttering on the buffered video or rendering jagged, pixelized images from lower-res clips). 320x240 was just a convenient (50%) scaling factor of the 640x480 VGA display.
In 2009 all U.S. t.v's will be digital -- and it's a pretty sure bet that media center type "Hi-Def" hybrid computer / t.v.'s will be more and more common -- meaning web pages layouts will likely be designed to fit the 16:9 format.
Yes, I'm rambling -- but with the time it takes to produce video and the speed that technology advances, I am afraid that all the 320x240 clips being published now will give sites an "out of date" look very shortly, (and they will not scale up to full screen on the newer 16:9 hardware).
The second "TED" video (listed above) is saved at 520x293 --- my question is; Does anyone have other 16:9 video sizes they're using or thinking of using?
My feeling is, that now is the best time to start publishing, (or at least readying yourself), for the 16:9 aspect ratio electronics world.