The 20:1 rule of thumb is very rough; much depends on the nature of what the hosts are doing. I.e. how much traffic is being passed back and forth and how time sensitive it is. Also, normally as you increase host edge bandwith, since the host's actually traffic amount usually doesn't increase with edge port bandwidth, you don't always need to increase the uplink bandwith (e.g. 48 FE ports with gig uplink that's performing well will likely perform just as well or perhaps even better with 48 gig ports and gig uplink).

As another poster noted, VoIP bandwidth is best dealt with using QoS, although it too has its own bandwidth requirements that depend on the codec being used and how the users are using it (e.g. normal business phone usage vs. sales/help service/support phone usage).

Etherchannel is an attractive option for increasing link bandwidth but it doesn't really perform the same as a link of the same bandwidth. For instance, assume a dual Etherchannel provides, on average, 50% more bandwidth not 100% more bandwidth. (NB: if channel hashing isn't adjusted for the link ends, Etherchannel might not even use its multiple links.)