do I need IM, VOIP or video conference?

I am thinking of getting web cams for my family for Christmas so that we can do real time video and audio "calls" to each other for free or very low cost. It looks like IM software like MSN Messanger or ICU might work or Net Meeting or a VOIP service like Skype. All say they have video and audio support but none are real specific, like does it do video and audio at the same time.

I don't do chat rooms or have needs for shared files. I just want to find out what the most stable system would be to make video/audio calls to my family. I am on ME with DSL. The other family members are on XP but some have DSL and some have dial-up. I could get XP if that made a big difference but one person has no way to get DSL. One person is in the Bahamas with DSL, another is in the UK with Dialup. I need something easy to use, since I will be loading the software on Grandmas computer and she is not what you would call computer savy.

I guess I do not understand what the difference is between IM, VOIP and video conferencing software for what I am interested in. What am I missing? Which of the three technologies should I focus on? What products would anyone suggest as being stable and working for Dialup as well as DSL? There are just too many products out there to try them all, so I was hoping to find someone who has used one and would recommend it for my needs. Thanks for any suggestions.