Does it have to be 5.1?
You can get some really good deals on 7.1 systems.
I picked up a ONKYO HT-S7100 for around $630 a little over a year ago that I'm really happy with.
New Egg is running a good sale on some systems right now.
Newegg.com - ONKYO HT-S6300 7.1-Channel Home Theater System With iPod Dock