Intercontinental calls with Skype are the calls that fail the most in my experience of late. Not sure if that is only the case in South America. National/international Skype calls with neighboring countries are quite ok.
The Skype executable takes about 200MByte to 250MByte of RAM when idle or in text-only chats. At least my portable version of Skype does. It also barely claims CPU resources. With modern computers you will hardly notice any slowdowns or other negative things when Skype runs in the background. But I also think that all modern chat/call software has no real problems in these regards.
But even if you find the application that is the most efficient, getting anyone else to use it...that will be your real problem. Which is also why the current big names in this area are so entrenched. There is a lot of inertia in this particular part of computing.