I tried a isight...and i dont think it is that good !
If you were disappointed with the iSight, you're really gonna be disappointed by just about every other webcam out there, regardless of whether it works with a Mac. I can't speak for the standalone iSight that's no longer produced, but the built-in iSights on the new Intel Macs are awesome! My co-worker has a webcam for her Windows computer, and the quality is atrocious in comparison to my built-in cam!
Webchats with other Mac users using iSights (like my wife and brother-in-law) are about as close to being there as the multi-thousand $$$ corporate video conferencing as I've seen. I was even able to get her brother's Apple TV working with his wireless network using iChat AV to see what was on his screen. That would have been painful, and only about a half-step better than just the phone with any other webcam.
Of course, if you don't have a Mac with the built-in iSight, your choices are limited. I would see if you can get a Firewire camera; that is more likely to work with a Mac. I don't know anything about them, but if you can find one (and I think the standalone iSight, expensive as it is, is a firewire cam) it might work.