Here are the main differences I was able to track down between the 605 and 606:
606 extras over 605:
- 4 HDMI inputs
- An extra component video input
- Video scaler, capable of 720p/1080i output
- Audyssey Dynamics EQ*
- 10W more power all round
- New remote
- HDMI CEC system now compatible with Panasonic's Viera Link, Toshiba's Regza Link and Sharp's Aquos Link, allowing the receiver to give one-button control of suitable TVs from those brands
* Audyssey Dynamics EQ works in conjunction with the Audyssey 2EQ system to adjust the balance of the sound to compensate for changing volume levels, giving a more consistent sound when listening at low volumes.
If these are worth the price difference, then the 606 would likely be the better choice. If not, you might be able to save some coin and have most of the options available on both.
I do believe component can be displayed over HDMI on either of these Onkyo models. As for the Yamaha RX-V663 being able to, I would assume so, but I'd check Yamaha's site to verify this.
Denon would definitely be a great option, however for a similar feature set as the Onkyo and Yamaha models discussed above, Denon prices themselves way out of reach. I've owned (and still own one) many a Denon receiver and do appreciate their build quality, but when it comes to price for performance, current model Denon AVRs are just way too pricey... -TD
EDIT: yeah, what Glocks said...