Last question first. Since you are planning on spending around $3K, make sure Samsung will honor their warranty if purchased from eCost.com. Many of the major electronics manufacturers will only honor their warranty if the item is purchased from an Authorized Reseller. Also, if you buy the reseller's warranty, make sure you know who will actually be sending a service-person to your home if you need service on your Sammy.
I bought an HLN507W last July and I'm very pleased with the set so far. I did have a noisy color wheel that was replaced under warranty last fall. Bigger is better if you have a greater viewing distance available. By the Sony guide to home theatre, you should view HDTV from 8'-6" on a 50" diagonal WS and 9'-6" from a 56" diagonal WS. For each of these distances for HD add two feet to watch SD. Why? Because SD has fewer pixels and the PQ look pixelated on screens that large at that close a distance.
My Sammy does a good job of displaying SD; however, that is via DVI from the E* 811 receiver I currently have, and I have the whole system controlled through a Monster HTS-3600 Power Center. When I first got the Sammy, my reciever was a 2700 w/ only S-Video available and I was using a basic Monster Home Theatre surge protector. The SD was at best adequate on some channels, and was almost unwatchable on some others, like locals via E*.
