If it were me I'd buy the best 3.1 system that I could afford for right now. The front left and right are doing most of the work for music, and the center for movies. Later when money allows you can pick up the surrounds and rears. You can also keep in mind is an upgrade path. For example the subwoofer that is now in my home office started out in my family room, then migrated to my bedroom, and finally my office as funds allowed getting better gear.
Here's an idea... Frys.com has
Polk R50 towers on sale for $70 plus shipping. Shipping is usually cheaper if you buy them separately. So that's roughly $180-200 shipped for a pair. Add an acoustically matched
Polk CSR center for $150 shipped. So now we have the 3.0 of a 3.1 system for $350 delivered.
So now we have $350 left for a sub, and for that you could either buy a
TSC T300 subwoofer; or by going $50 over budget get a TSC T250 sub
and a pair of
Polk TSi100 bookshelf speakers to use as surrounds. Now this won't make audiophile system of the year but it's probably the best possible bang for the bucks available, should kick butt on any prepackaged system at that price, and should look pretty good. I picked up a T250 for my bedroom and it's decent. It's SVS but it's very good for the money and has gobs of power for a medium sized room.
Just based on my reading I'd cross over the mains at 80hz and I think you'll be styling.
BTW if there is a Frys near you then I strongly recommend listening before spending money on speakers. What sounds good to one person can be torture for another.