At your budget price. I recommend you look at the speaker company bookshelfs and a Dayton 12 inch off ebay(they go for 90 usually)
You could also mimic some of my setup. I put together most of my system for around 500(until i upgraded receivers)
But what I suggest is that you actually just get an old used 5.1 receiver off ebay. 80 percent of your budget should be on speakers. The sub should take a good portion of that. But at your lvl the Dayton 12 would probably be your best option.
So only spend up to 100 on the receiver
maybe 150 on the sub(if you order from parts-express)
that leaves you with 250 left
for that you will want 5 speakers
http://www.thespeakercompany.com/LCR3-P-Series-Home-Theater-Front-Speakers-P41C6.aspx order and try those. They have a 30 day money back and if you don't like them.
Now you have 100 left
http://www.thespeakercompany.com/P5-PR-525-2-way-Bookshelf-Speaker-Pair-P42C6.aspx
Those will probably work just fine for the back.
Ok that's about 500 dollars.
Now go find a decent used receiver for under 100 on ebay.
Now if you want to have a great system. I suggest you start of 2.0 with your budget by getting two towers and .
Then add a subwoofer, Center, Sides, Rears as you get the funds.
Then you won't be replacing you will be adding.