Hey, welcome! What lenses do you have? What do you think is limiting you?
I would suggest you get something like the DA 50mm f1.8 lens. It doesn't cost much, but it has snappy AF and great image quality! And it works well in low light situations, too (just switch to f2.4 or lower, and you can handhold it indoors with nicely sharp photos). This is something great that Pentax has, the DA 35mm f2.4, DA 40mm XS, and DA 50mm f1.8 lenses - very cheap, but amazing quality! Lots of bang for buck. Of course, you can look at higher end lenses, too, but those are over $500 easily.
I think getting a lens like this can be an important first step in going from "decent" photos to "great!" and even some stunners
Edit: If you feel you need more MP (for large prints, like posters), or faster AF, then you might want to consider a K-5II. These are on firesale right now (discontinued, last season's model), but they have pro features and great AF, great image quality. Definitely a step up from the K-x, but it is also a little more complicated to use. And you need decent lenses to get the most out of it. With DSLRs, lenses are often more important than the camera body.