Manual for 606 specifically states: "In addition, video signals from a PC are not supported". (page 31 of the 606 manual, "Making HDMI connections" chapter, in the Notes.
Newer graphics cards/chipsets, paired with newer drivers, provide full HDTV support, meaning that outgoing video is a proper 720p, 1080i or 1080p, at a proper frequency, with overscan if needed, which AVRs can understand. You definitely should try getting the latest Catalyst supporting your card model, and see if it includes proper HDTV support. If your card is a PCI-e, not AGP, you have lots of options, best probably being a Radeon HD 4650 card.