If you are looking to do this in an office set-up then bandwidth is the key factor. Equipment all works to the same standard and so is compatible with all interfaces. The better the bandwidth, the better the quality of the picture.
If you want to have multiple parties together it is called 'bridging'. For occasional use you can use a commercial company to host the call. Each party dials in to the host and can then conference with all others.
If you are going to host calls you can have a bridge on your premises. This is a trade off between short and long costs.
Polycom are certainly one of the main companies, also Tandberg are well worth looking at.
I cannot give yards of free advice as I am a freelance telecommunications consultant!