I was thinking about buying a 300mm FD lens for my GH1 - this, as far as I can tell, is 600mm equivalent when placed on the FD? Meaning, that a Telephoto 100-300mm f/5.6-6.7 Manual Focus Lens would be functionally equivalent to a 200-600mm f/5.6-6.7? Was thinking about picking one up for sports photography.

Your maths is spot on if the 4/3rds GH1 has a 2x factor in its chip size relative to the standard 36 x 24 mm frame.