Try Skype. I have started using it for video calling family who have PCs. It works great, and with a very nice frame rate. Video and audio quality are excellent. It works as both an IM agent and an audio/video call agent.
Skpe is free for use between any two Skype equipped computers, and it has clients for all of Mac, PC and Linux. So it is a pretty broadly based package. I am very impressed.
For the record, I am not associated with Skype in any way, just a really happy user of their product.