Seburn
Voip in general is pretty latency dependant. There are basically two issues with latency, firstly the latency itself and secondly the variation in latency (jitter). Humans can not effectively "hear" latency of less than 250ms, so if you have a latency of 250 or less you will not notice any "lag". Speech is quite acceptible up to 500ms and with a bit of practice and someone you know quite well you can have a conversation up to 1000ms. Past that and you will be talking over each other.
Jitter is another problem on its own. Obviously the voice packets do not arrive at the same time, your voip protocol has a build in buffer usually 40-50ms worth of buffer, this obviously adds to the "lag" but allows the packets to be reassembled in the correct order. If a packet arrives outside its buffer period it will be discarded, you will then get breakup - similar to a cell phone in poor coverage area.
So to sumarise low latency low jitter = perfect voip
high latency low jitter = acceptable with some lag
high latency high jitter=forget it
Wrt skype if you are using skypeout the the quality of the pstn gateway that you will be using is also critical. For example skypeout to new zealand very acceptable skypeout in australia can be quite dodgy. I think that the exponential growth in skype has put some strain on their gateways with some more overloaded than they should be.
Their is an interesting site.
www.testyourvoip.com which will test your connection and give you a score out of 5 as well as some info on your latency and jitter etc.
On IB i average about 3 which results in quite acceptable conversations. On isdn (storm) i can get up to 4.
Hope that helps. Dave