Here's my 2 cents.
I know you want the immediate impact of a 5.1 system, but that may not be the best option given your budget. Putting together a 5.1 system on that budget will only satisfy you in the short-term.
I'm going to suggest that you start with a 2.0 system. Get some decent speakers for $500 (look around and wait for sales). You'll be able to get $1,000-$1,500 speakers for that price if you are patient. And trust me, a 2.0 system with good Fronts is outstanding.
Later on, when your budget allows, add a Centre Channel or a subwoofer (whichever you prefer) and go from there. Don't worry about 7.1 for now, there isn't enough out there to fully enjoy it anyway. You can do that later.
Build slowly with quality componants. That is the best advice I can give you!
John