For Windows XP: If you have been unable to get AC3 or DTS passthrough via your ATI Graphics card with HDMI try this official ATI driver:
ATI Catalyst™ 9.4 HDMI Audio Driver for Windows XP Professional/Home Edition
The Catalyst™ Software Suite states that the HDMI Audio driver is supported on the Windows Vista OS only at this stage. Its never straightforward, especially with the HDMI driver hiding on the AMD website.
Here is the Realtek ATI HDMI Driver, this may also help.