Here's what I have been able to do to try videoconferencing on my iMac:
I bought two USB Connectix QuickCam VCs for my sister and I. They retail for $79, but I landed a sweet deal early this year when Office Max advertised a $50 off coupon. So I got both for $30 each.
And I have been very pleased with the performance by the camera. I had to download the drivers off the connectix web site as they were not included on the CD.
Software is another matter. Being cheap, I have tried the shareware, including iVisit.com, cuseeme, and ICUII. They were all alright, but I was very limited by the fact that my sister has poor phone lines. Her connection rates are around 24,000 bps, while I have a more comfortable 49,333 bps. The video and audio were never too good, usually around 1 frame per 5 seconds.
There is other software you can try, including the new CUSeeME by White Pine software and ClearPhone, but they cost about $80 each I think.
Bottom line though, make sure your connection speeds are up to par to make this feasible. I am still quite happy with the QuickCam. My sis sends me pics of my new niece in every e-mail.
Hope this helps.