Original poster - Congrats on your find of the Monitor 70's and CS1 center for $300. Thats a killer deal. I'm sure you will love them. I bought mine new on Amazon and payed a total of about $400 then spent more on surrounds. I have two sets of Polk RM8 bipole Wide Dispersion surround sound speakers ran in my HT as rears and surrounds with my Monitor 70's and CS10 center (only difference is piano black finish vs standard black veneer). Here's a link to them...
My room is about the same size as yours, 21' x 11' x 8.5' high and I am powering the speakers with an Onkyo TX-NR709. I love the way my setup sounds and for around $500 for the 709 I think you will be hard to find a better receiver in that price range. My vote definitely goes to spending a little more than $300 on the receiver. You will most likely want to keep it longer than any other component in a HT system. I cant say enough about the 709 except to say that if you are really thinking about getting any new receiver... DO LOTS OF RESEARCH. DO LOTS AND LOTS of it. Then do some more. It took me 3 months of reading on here and amazon to decide which receiver I wanted. Features are what distinguishes most from others. You will find that as far as brand vs. brand it is VERY negligible. Some people like Denon, some Onkyo, and some Yamaha. You will most likely have to have purchased all 3 at some point to decide which ones you like or dislike more than others. I personally am an avid Onkyo fan. I currently own 3 Onkyo's. One is a 2004 model that is 7 years old (TX-SR302), one is a 2009 model that is approx 2 years old (TX-SR607), and my 2011 model (TX-NR709) I purchased in March. All still run great and have never had any issues of any kind. Other people have had different results. I have also owned a Kenwood, Sony, and Yamaha receivers. Not that I didnt like them, but I loved the Onkyo's more. I have never personally owned a Denon but everyone RAVES about how nice they. Comparitively, I found the Denons to be MUCH higher priced per what features you got VS. the same aprrox level of Onkyo's until you got into the 100x, 200x, 300x models of Onkyo's where the price and features were higher for Onkyo and lesser for Denon. If you want to go truly budget and still get a decent recevier I would say go with one of the Pioneer VSX models... you can get the VSX-1021 or 1121 which are their tops in that line respectively for around the prices your are talking. BUT if you are truly wanting something to be future proof and to last you many many years I would say go with Denon, Yamaha, or my suggestion the Onkyo TX-NR709.
Here's a link... but wait for it to go on sale somewhere or find one used. I payed just over $500 for mine back in March before the 2012 models really hit the mainstream. I would think that during Christmas you will probably be able to find a refurb or used one for $450 or less if you're patient.
As far as my speakers, the RM8's were about $100 a set so definitely more than the Pioneer speakers but I can guarantee that they will sound better AND you will get more room emersion/fill with bi-pole speakers than standard bookshelves. That can make a big difference when listening to surround and rear speakers. The price fluxuates A LOT on the RM8's leading me to believe that they are nicer than I had originally thought. I've seen them range on amazon from $100 all the way to $200 a pair.
One thing I really love about them is that they come with wall mounts so no need to buy them. Another is their size. Their footprint is super small for the sound they give out making them highly desireable for people (like my wife) who are concerned with not only SQ but asthetic appearance as well. I have considered upgrading my RM8's and moving them upstairs for my wife. I highly suggest them
I'm looking at Polk Audio FXi A4 or A6's or these Fluance Bi-Pole surrounds:
Here's a link to the FXi's... although they are quite pricey and out of most people in our budget level's expense range... I am seriously thinking about making the jump but I think an external amp is in my future prior to the surrounds.
While were on the subject of this speaker setup, I do have a question about amps. My Onkyo NR709 is powering my Monitor 70's, CS10 center, and RM8 surrounds right now but I would like to get more and/or as much as possible out of the speakers. So I'm thinking about getting an external amp or maybe two. Which one(s) do you guys recommend and what kind of cost am I most likely looking at? I'm thread searching for similiar discussions right now but haven't found anything yet.