you can go to dakmart.com for the Denon, and Accessories4less.com for the Onkyo. I bought my Marantz from a4less, and I recommend them. I really don't think you could go wrong with any of these products, there just isn't that much differentiation unless you aren't getting hi-def decoding or room correction. If I remember correctly, the 2808 has a pretty decent one.
Don't worry about the video processor in receivers. They only matter if you have a crappy DVD player or VCR player plugged into your receiver. Most average or better blu-ray/hd-dvd/dvd players will offer better vp performance than a receiver. I wish they'd just strip them out of receivers.