under $500 used which a/v receiver or processor

There are pre-HDMI Cambridge Audio AV receivers getting blown out in this price range. Try Audio Advisor's site for deals on them.

Rrekstad ;
This is from Onkyo's website :

TX-SR605
7.1-Channel Home Theater Receiver

HDMI V1.3 repeater (2in/1out, 1080p compatible)
HDMI video up-conversion with DCDi technology
Dolby Digital Plus, Dolby TrueHD and DTS-HD decoding
90 w/ch x 7 ch

What audio formats are you refering to ?

any opinions on the marantz sr 4001 in terms of 2 channel music

ROTEL RSX 1055

Rrekstad, as indicated by Saki70, the 605 does in fact decode Dolby True HD and DTS HD Master Audio.

http://www.onkyousa.com/model.cfm?class=Receiver&m=TX-SR605&p=i