Thanks for the suggestions, but I am not considering incremental purchases, or spending much more than $100. I've already spent $170 on the receiver, some cabling, and speaker stands, and I'll probably also need to get a stand for the center speaker (if I get one).
Right now I have a receiver, four Klipsch computer speakers, and no sub. I am satisfied with this - it's much better than my TV speakers, which was the original goal, but I also know I can do better. If $100 can improve this, I'd like to spend it where it will count the most. If I can't improve on this meaningfully without $350-$500, then I won't spend a dime.
I see the following options:
1: Spend $100 on a sub and/or center speaker, add them to the existing 4.0 Klipsch configuration
2: Sell the front two Klipsch's, spend the money plus $100 on three better front speakers, keep the other two Klipsch's for surround
3: Sell all four Klipsch's, spend the money plus $100 on a complete 5.1 set
4: Keep the hundred bucks