When it comes to video conferencing the primary determinant of picture and sound quality is the capacity of the "pipe" that joins the two sites.
A commonly quoted magic number is a capacity of > 384 kb/s (kilo bits per second) in each direction, with more being better. So if you have a 2 Mb/s (mega bits per second) ADSL line you should be right? Right? Perhaps. ADSL stands for Asymmetric Digital Subscriber Line. The asymmetry refers to the fact that the download speed is much faster than the upload speed.
Most of the time this does not matter as we do a lot more downloading than uploading, however for video conferencing upload speed is critical, because it is upload speed that determines how good the picture will be at the other end of the connection.
You can test your connection using one or more of the speed testers here.
So how much data do you use? 1 Byte contains 8 bits so 384 kb/s is close enough to 50 kB/s (kilo bytes per second) - and we use that much data in 2 directions making a total of 100 kB/s. 3600 seconds in an hour gives us minimum use rates of 360 Mega Bytes (0.36 Giga Bytes) per hour.
Although it depends on the software/hardware configuration most video conferencing equipment will happily use substantially more data than this (unless specifically configured not to). A data use budget of 0.5 Gig per hour is a good real world ball park.
The way cheaper ADSL providers manage to be cheaper and still make a profit is by selling the same chunk of capacity to more people. This is called the contention ratio.
If you have wireless ADSL at home and have kids you will have seen contention in action as you connection grinds to a halt while your kids simultaneously stream music, Skype friends and inbox frenemies on Facebook - they are competing for limited capacity. If you use the same provider as your neighbour it might not even be your kids slowing your connection down.....