Hi Stewart,
The Decklink HD Pro single link card is capable of software down conversion and cross conversion on Mac OS X. Software down conversion and cross conversion is not supported on Windows at this time. There is a support note regarding Down conversion with DeckLink cards on our web site. It is pretty long but it does detail which features are implemented on Mac and Windows.
The only card currently supporting down conversion on Windows is the Decklink HD Pro dual link card as it includes hardware down conversion. If you have any other model of card, the down conversion option will be grayed out.
If you are using a PowerMac G5 and this option is grayed out, then something strange is going on. It could be that your DeckLink HD Pro card is sharing the 100 MHz PCI-X bus with a slower PCI card. This would slow the whole bus down to the speed of the slower card and would disable the HD functionality of the DeckLink card. However you could still use it for SD work and for playing out HD video as SD.
Regards,
Luke Maslen
Blackmagic Design