Road Cycling: If I were upgrading from a commuter bike to a (flat-bar) sport bike, what should I consider? Any good options?

Honestly, I don't know that you'd gain much by upgrading. A friend of mine used to do 20-30 mile rides with me on a Jamis Coda, which is pretty similar to your current bike. In my mind, tire width and easy gears are the most important things for riding any kind of trail. I've done some nasty trails on 32 mm wide tires, just like what you have on your bike now, and you certainly have good enough gearing. Two of the links you posted are dead, but the biggest difference I can see between your current bike and the Specialized you linked to are the drivetrain components. Even there though, you aren't getting a huge upgrade. The shifting might be a little nicer on the Specialized, but you would actually have a smaller range of gears than your current bike (your 'easiest' gear is easier on the Cannondale). If you are riding on flat terrain, that won't matter, but if you need to go up hills, or if you are really tired during the ride, you may wish you had them.

I'd consider a bike you can use for all sorts of things.  I ride a Cotic Roadrat as a day-to-day bike.You can turn it into pretty much anything from a cyclo-cross bike to a commuter to a fixie to a racer as your mood takes you or if you want to try out different things.  Awesome bike. http://www.cotic.co.uk/product/roadrat

You haven't said what you expect to get from your new bike which you don't have with the Quick 4. Speed? Durability? Better fit? More carrying capacity? I used my $300 commuting bike for many 60-100km rides. It fit and I liked it so I rode it until I started taking multi-day camping trips.