Have you ever heard any Polk speakers? All speakers tend to have their own unique sound. The best way to make this type of decision, is to go to a local store if possible, and listen to as many types of speakers as you can.
The Rti 10's and monitor 70's are in different speaker lines. The RTI line is a step up from the Monitors. Having said that, I don't know which speaker will be better for your application. My instincts tell me the 10's. I've listened to them at a local dealer 2-3 years ago, and I liked them. I bought RTI8's for a family member, but would have loved to get the 10's. Lack of $$$$ didn't allow it though. The 10's will most likely play a bit deeper, but I'd verify that with another source if I were you. I don't know a whole lot about the 70's. Many users like them, and for the price, you almost can't go wrong.
I have heard the Csi3, that's what I paired up with the RTI8's. I'd recommend getting the Csi5, if you have the space for it. It's huge, so take a look at the dimensions before you decide.
The sub, I don't know much about. I've never been a huge fan of polk subs, at the 505's msrp, you could get more bang for your buck with an Internet Direct manufacturer. Look at Epic, elemental Designs, SVS, HSU, to name a few. You'll spend more $$, but you definately won't regret it. Considering the polk models your looking at are heavily discounted, maybe you're not looking to spend too much cash.... if not, the 505 might be alright for now.
Your onkyo receiver isn't bad, I wouldn't rush out to buy a new one just yet. If it has pre outs to connect to an external amplifier, then your all set power wise for the future. It does lack the latest audio decoders, dts hd, truehd. I'd upgrade it last though, there's no rush there.
Personally, I'd go with the 10's, the csi5, and a different sub altogether if I could scrape together the cash.
And don't forget, listen as much as you can before you buy. There are hundreds of speakers to choose from out there. Having said that, you are going to love pretty much anything you choose, it will be a huge upgrade from your onkyo speakers.
Good luck, and sorry to sound like such a fence sitter as far as the speakers go. I don't want to push too much of my own tastes onto you...
I should mention that I am a polk owner.
CSIa4 (currently shopping for a CSIa6, which is the newer model of the 5 you are looking at)
SVS PB12-nsd sub
You can see why I try not to sound too biased in my info to you...