Changing your speakers is the biggest difference you can make in your system. With the budget you stated, there are plenty of options out there. Grab your favorite CD and head on down to some of your local hi-fi shops and listen to whatever you can.

My personal favorite experience with a speaker in that price range would be the Sonus Faber Concertino. It really made my jaw drop with the size and depth of the soundstage it produced. You might, if you have the space for it, look into Magnepan speakers. They've developed a pretty loyal following so they must be doing something right. Their entry level speaker, the MMG, is available in floor-standing form for $550 shipped directly from them with a in-home trial period.

Good luck and have fun!