oops - I thought that you were already using the Realtek
driver for the ATI card, I could have mentioned that.
I have been thinking about what I had written and as my amp does not have a HDMI input when I connected the HDMI cable from my 4870 to my TV the optical connection from the TV to the amp downsampled the audio from 5.1 to two channel stereo. So even though the HDMI cable might well have carried the 5.1 signal it was not delivered to my amp as that by the TV.
Using s/pdif, of course, resolves all of those issues.
BTW - if you right click on the orange Realtek speaker icon in the system tray and select "audio devices" have you selected the "Digital output device" and selected the appropriate settings within that (DD / DTS audio etc etc etc)...?