Yes u can however there are some problems regarding the 4xxx series and Yamaha/onkyo revievers, they dont seem to recognise the EDID properly so u need to overwrite it. I did this with my 4870x2 however i only gotten stereo out of it. Somewhere here there is a thread regarding this EDID overwrite stuff. If u google "HDMI sound 4870 forum" cant remember the thread. Inthere u can find evrything u need to know about HDMI out of ur GFX card for 4870x2.
Myself i would not advise it since its only stereo. I spend alot of time working on this and went for the spdif in the end.
Hope this can help u