depands on brand and features. Most of the digicams do record movies as well but just few of them allow some kind of usb-stream. My sony dsc P8 doesn't work as a webcam.
it's not really called usb streaming. I made up that word
You should check the features of the digicam. If you can use it as webcam, it should be mentioned. I am not sure, but those hp digicams have many additional options.. Take a closer look
why not just pick a cheap usb webcam? Check out ebay! They are really cheap..
Best would be a firewire webcam, if you don't want to deal with some third party drivers and licences. I got mine for approximately $10 (D-link 350C)
I have a PowerShot A80 and it comes with a software (I think the A75 too) that let's you remote control your camera from your computer.
With that application you can see what you would see with the camera's lcd screen in realtime.
I think if that app can do it then any software could, but OS X doesn't recognize it as a webcam.