The answer depends on the legacy PBX, what interfaces it already has, and how much free capacity it has. If it supports SIP, using G.711 codecs and the relevant SIP components are actually installed on it, you will need no additional hardware or software on the Asterisk machine, besides Asterisk and DAHDI.
You really need to read Asterisk: The Future of Telephony, which cand be found at asteriskdocs.org/