Yeah there are TWO realtek drivers. These are for different audio devices. "Realtek ATI HDMI" works for the HDMI audio on Radeon 5000, 6000 cards. The other Realtek driver is for built-in motherboard analog and digital (SPDIF).
I'd just stick with the Catalyst AMD HDMI driver. It works for me. I have bitstreaming TrueHD and DTS-HD working with MPC HC and ffdshow.