Even on higher bandwidth links you can run into problems with VoIP and video conferencing applications, when other applications are competing for the same data link. Even though the actual total throughput required may not necessarily be that high, it is very latency sensitive.
Hence you might have perfect chat / video quality until one of the computers decides to check for email, or someone has a quick browse of a web page. That can be enough to introduce a glitch in the flow of data that is visible or audible.
To fix this properly, you need a router that understands concepts like bandwidth reservation, and tuneable Quality of Service bands. So that it can recognise these real time protocols being run through them, and prevent other applications having their packets gain priority over the real time ones.