VoIP - tips?

Hi, can anybody give some valuable tips, recommendations, comments, experiences regarding switching a mid-size organization to VoIP? We’d like to go in this direction and are currently evaluating what’s available, what scope we’d like to go for (international, national, internal), what system set-up to use (purely PC-based, using the existing internal telephone system) etc… One aspect I’m not too clear about is the usability of this technology with third parties that are relying on normal telephones … any input is highly appreciated, thanks a lot, Xpet.