nellie7799,
Let me start by saying I have no experience with either the SR7400 or SR8400 receivers, but I do own an SR5600. I have had it now for about 4 months. I have to say that I have really enjoyed it. It has a lot of very useful features. I have found the upconversion features of the 5600 very useful, but I do believe that the two others you mentioned do it too. From what I can see, the two others you mentioned can do everything that the 5600 can do, but have a few more features, some of which are just more input jacks, better remotes, and the 8400 seems to have a toroidal transformer, where the others don't seem to. None of them are THX certified, I think you would have to go to the next model up to get that, meaning the SR7500, and SR8500.
You can do some comparisons with the following links:
SR7400:
http://www.marantz.com/p_product.cfm?id=2544&cont=u&line=rcv&cat=hf
SR7500:
http://www.marantz.com/p_product.cfm?id=3273&cont=u&line=rcv&cat=hf
SR8400:
http://www.marantz.com/p_product.cfm?id=2662&cont=u&line=rcv&cat=hf
Or just go to the main receivers page:
http://us.marantz.com/Products/172.asp
In any case I will recommend the SR5600 without any hesitation, because it has been absolutely wonderful for me. I am using a pair of Paradigm Titan's for my mains, with a CC-270 for a center channel. It has just been incredible! I am sure that the AVR models above it would be just as good and you would be happy with any of the models you chose, however as a personal preference, I try not to bother with refurbs. That doesn't mean that they are bad by any means, I just like them brand new. I am sure others out there would say go ahead with the refurb, whatever fits your budget. You may want to make absolutely sure though that you can get the 3 year warranty if you decide on the refurb, if not, stick with the new model. With the way these Marantz's are built, I doubt you will ever need the warranty, but it's better to be safe than sorry. Let us know what you finally decide on.
-capT