Network Intrusion Detection/Prevention Systems (NIDS/NIPS) are there to protect the wire. HIPS is there to protect the system. Signatures/rules of NID/PS and HIPS vary a bit. They monitor different for different types of activity. Once an attacker has access to a system, the NIDS/NIPS may not capture things like "change system IP" or "change admin password"; HIPS would catch this.
NIDS/NIPS should be at important bottle neck points in your network (of course you want the hardware to match what crosses the wire (e.g. 1 or 10 Gig throughput)). For example; at the edge of a particular branch or office. Or at your enterprises gateways.