Keep in mind that for the Logitech to work with ichat, you have to buy additional 3rd party software, because ichat doesn't support usb webcams. Firewire only. So there'd always be the chance of future system software invalidating the 3rd party ichat-enabling software.
Basically, you have to work your way backwards: figure out who you're going to be communicating with, and what software they're going to be using. Do you need cross-platform? That necessitates mac-compatible software. when I was looking into this, the only thing I came up of any quality as good as iChat with was iVisit. The free version has a way-too-small video window, to get anything bigger, you have to pay. So, if your usage demands such software, add the figure of the software to the cost of the usb webcam. How close do you come to the price of the iSight?
OreoCookie: When I was looking at Yahoo Messenger for the Mac, I seem to recall that it supported video but not voice? Is that the case?