Here is my 2 cents...
1. iSight just works and it works well, so it is worth the price (also, a firewire hooked up to a Camcorder works just as well).
2. A usb cam will work on a mac, but it is not so simple.
a. You need to get a driver here (
http://webca...ceforge.net/)
b. You need to purchase this application to get it to work with iChat (
http://www.e...ichatusbcam/)
iChat works great with other iChat clients. iChat is a little buggy with AOL IM (it is not officially supported), and you will spend a lot of time troubleshooting the aol side of things. Skype to Skype video chat works as well as iChat to iChat. The nice thing about using Skype is that you can go Mac to PC with no issues.
If you are planning on using an eMac, it will probably work fine, but you may get some lag. I found that Skype video chat pushes my 1GHz Powerbook processor to about 80% to 90%. iChat seems to run at about 70%.
Good luck. It can be difficult. I have been gradually setting up my entire family (mac and pcs) with video chat, thus I unfortunately have more experience in the matter than I want to admit.