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The Denon AVR 790 is a pretty good choice for an entry level AVR. The only drawback I see is the fact that it doesn't have pre outs for adding an external amp. May not be a huge issue, but if you want to make your system a little more "future proof" I would suggest you look into one that has pre outs, without them, you don't have the option of addin an external amp down the road. The RTi's love power, but the 790 will do a good enough job of powering them. If somewhere down the road you wanted to give those RTi's more power, you would have to upgrade your AVR to one that has pre outs, before you could add an amp. The AVR 990 has them as does the 3310CI and above. They are a bit more pricey though, the 990 strats out at around $900. There are cheaper options if you look at other brands like Onkyo, Pioneer and Harmon Kardon though. Any more questions feel free to ask.
I am pretty sure it would sound ok. Most people recommend the A6 since it is a larger speaker and 50-70 of home theater goes through the center. I have the A4 due to size restrictions. The A6 is huge and i could not fit it in the cabinet i have. Overall i think the A4 would be fine. When you calibrate the system you may have to turn it up a little but thats cool.
I know that i was looking at the A7's and the A5's. After asking around a lot i decided to go with the A5's. Here is a thread about my decision. Most people say either get the 5's or 9's. Not that many recommended the 7's. I am sure that they are great speakers, i have heard them once, but i went with the 5's my self.
No problem, just glad I could help. You will be ok with the CSiA4 for your center with the A7's, just may have to turn up the volume on center if the sound isn't good enough. If you can afford the A7's I would get them, if you get the A5's you will always wonder what if. The 125 is a beast, I was surprised by it's size when I got mine.
Sounds like a good start to me, the only thing I would suggest is maybe stepping up to the CSiA6 if you can fit/afford it. The sub looks like a good option as well, only one I have experience with is my PSW125, which I like very much. Good luck, and if you have anymore questions, feel free to ask.