Express Talk (and all of NCH's other VoIP products) support two main protocols: G711 and GSM
G711 provides very good quality, but it uses a lot of bandwidth (64 kbps or more)
GSM provides good quality and uses much less bandwidth (13 kbps)
The thing is you will never get the full bandwidth you are allocated, especially when you're talking about an external ISP. For instance, I have an 8.0 mbit internet connection, but on average I can only use up about 4 mbit of bandwidth at a time due to external factors such as distance, quality of the connection, the amount of devices accessing the network etc.
So if you have a 256 kbps connection, even if your ISP was of a superb quality, and even if you lived next door to the telephone exchange where your internet connection is routed through.. you shouldn't expect to ever get your full 256 kbps worth, and should probably assume at any given time there is only about 128 kbps to work with.
However, if your modem/router supports QoS (quality of service) you can help prioritize VoIP call data so that it is guaranteed to get as much bandwidth as possible by limiting the bandwidth of other internet data (such as checking e-mail, or normal web browsing).
So what I'm saying is don't push it, you probably have a lot less bandwidth than you think. Tip: You can using speed meters available on the net (e.g. speedtest.net and the one at 2wire.com to estimate your actual available bandwidth).
Let me also add this: Most LANs have a bandwidth rate of about 100mbit, so you should have no problems handling several calls between users on your LAN. The thing to be worried about is several simultaneous calls to people outside of your local phone network. Remember that the bandwidth is only used when there is an active call (only very minimal data is sent when there is no call in progress). So in order to use up all your bandwidth you would have to have several calls taking place during the same time period.