[bhritchie]

The book I mentioned, The Shape of Ancient Thought by Thomas McEvilley, is an amazing one. That's the place to start. My now vague recollection is that the influence went first from India to Greece (via various other places) not as what I would call philosophy but maybe "speculative theology" or "speculative religion" (think Vedas, Upanishads) Then more fully formed Greek philosophy influenced Indian thought via the Hellenistic world following the Alexandrine conquests (so e.g. you wind up with Buddhist philosophy like Nagarjuna's Vigrahavyāvartanī that deploys some arguments familiar from sceptical schools of Hellenistic philosophy.