I would try to swing a Polk Audio speaker setup if you can. The sound is much better than Infinity, IMHO, and it can be cheaper if you look hard. Get at least a Polk Audio 10" sub, but I'd go with Velodyne 8"+ anyday of the week instead. I've got a Polk Audio 5.1 setup w/ a Velodyne subwoofer and am beyond impressed.
This is my setup.
Front towers: $149 ea. (discontinued, July 2000, Circuit City)
Center channel: $69 (open item, Circuit City August 2001, new for $77 at Fry's Electronics believe it or not!)
Surrounds: $99 (on sale at Crutchfield.com, September 2001)
Subwoofer: $199 (open item, damaged theft recovery at Circuit City, June 2002)
SPEAKER TOTAL: $670.00
I recommend you go through a few Circuit Citys in your area and look for the open box receivers and speakers- they will come with warranties, are haggleable, and are amazing deals. Definitly worth it. Make sure you run at least Monster Cable-grade speaker wire (100' spool at Best Buy for $59, less on eBay). On a personal note, I plan on buying a different reciever when I can afford it- the Sony is great for features, looks, and sound to the untrained ear, but the white noise SUCKS at certain volumes and ruins audio and DVD tracks.