New Yamaha Receiver
From a quick glance, the Yamaha RX-V659 includes the following new features:
* Burr Brown D/A converters.
* I-pod doc capability ($100 for doc)
* Nuro-surround (jack that plugs into your brain???)
* XM port with 5.1 capability.
* Costs more.
I have the HTR-5860 (same as the RX-V657) and would recommend it. You can find it online for about $300 from authorized dealers.
In the $500 range, I would also consider a refurbished RX-V2500 which would be a big power and feature upgrade from the models you listed.