2320 is a sourcebook for T20, not a stand-alone ruleset. It is based on the upcoming Traveller D20 Guidebook (or Player's Handbook, if you prefer) which does not require a D20 Core book.
So you need 2320 and the Guidebook, or 2320, the current Traveller's Handbook, and a D20 Core book.
The free D20 SRD contains most of what is needed, though.
Colin
2320AD writer