Well, when dealing with music production systems, it is required to use an external soundcard, wheter it's a PCI, Firewire or USB sound device (Usually from Terratec, Egosys, M-Audio etc.). These soundcards have very optimized ASIO drivers which brings down latency to just a few milliseconds. Then its possible to play softsynths in realtime with no noticable delay.
However, I actually don't know what's reqiured to get a VoIP system working optimally.